For load development, some guys shoot 3-shot groups. Other guys shoot 5-shot groups, or even 10-shot strings. But for testing its projectiles, Sierra Bullets takes it to another level entirely. A while back Sierra was testing its .30-Caliber 175gr HPBT MatchKing in the Sierra underground tunnel. The results appear above — a FIVE HUNDRED Round group!
500 Shots Form 0.82 MOA Group at 300m (328 yards)
Sierra’s trigger-pullers sent five full boxes of bullets down-range at a single target. The photo above shows the result of 500 shots taken in a 300 meter test tunnel. The raw group size, edge to edge of the farthest shots, is about 3.13 inches, as shown on the calipers’ metal linear scale. Subtract a .308″ nominal bullet diameter* to get the 2.823″ on the digital readout. So you’re seeing a 2.823″ group at 300 meters (328 yards). One MOA at this distance is 3.435″ so this 500-round group is 2.823 divided by 3.435 or 0.82 MOA (0.8218 MOA to be precise).
This 500-round group was shoot as part of a pressure/velocity test for a commercial customer. The cartridge was .308 Winchester, loaded at 2.800″. The powder was Reloder 15. A 26″ barrel was shot from a return to battery rest. The gun was cleaned every 125 rounds and two foulers shot.
What do you think — could you beat this group from a bench for 500 rounds?
One Facebook poster joked: “500-round group? Everyone knows anything less than 1000-round groups are a waste of time and statistically irrelevant.”
Sierra’s 300 Meter Testing Tunnel
Ever wonder how (and where) Sierra tests its bullets? The answer is underground, in a 300-meter test tunnel located under Sierra’s factory in Sedalia, Missouri. The photo above shows the construction of the tunnel back in May, 1990. Like most bullet manufacturers, Sierra does live-fire bullet testing of its projectiles. Sierra’s 300-meter test range is the longest, manufacturer-owned underground bullet test facility in the world. In years past, Sierra offered free visits to the test tunnel as part of a factory tour.
* Normally, to get an exact group size, you should subtract the TRUE bullet hole size, which is usually smaller than the nominal bullet diameter. E.g. a .308 bullet hole may show on paper as .298 or so. But here, for simplicity, we are subtracting .308″ because we do not have the original target to measure.
The anvil is the tripod-shaped thin metal piece protruding above the bottom of the primer cup. Getting the primer sitting fully flush on the bottom of the case primer pocket, without crunching it too much, requires some keen feel for the progress of primer seating.
Sadly, Glen Zediker passed away in October 2020. However, his insights live on through his written works. This feature is based on Glen’s popular reloading books and his articles for the Midsouth Blog.
In two informative Midsouth Blog articles, Glen Zediker offers helpful advice on priming. First he examines what happens to the primer itself as it is seated in the cup. Glen explains why some “crush” is important, and why you never want to leave a high primer. Glen also reviews a variety of priming tools, including his favorite — the Forster Co-Ax Bench Primer Seater. Then he offers some key safety tips. Glen provides some “rock-solid” advice about the priming operation. You’ll find more great reloading tips in Glen Zediker’s popular book, Top-Grade Ammo, which we recommend.
Priming Precision vs. Speed
Glen writes: “The better priming tools have less leverage. That is so we can feel the progress of that relatively very small span of depth between start and finish. There is also a balance between precision and speed in tool choices, as there so often is.”
Benchtop Priming Tools — The Forster Co-Ax
Glen thinks that the best choice among priming options, considering both “feel” and productivity, may be the benchtop stand-alone priming stations: “They are faster than hand tools, and can be had with more or less leverage engineered into them. I like the one shown below the best because its feeding is reliable and its feel is more than good enough to do a ‘perfect’ primer seat. It’s the best balance I’ve found between speed and precision.”
Load Tuning and Primers
Glen cautions that you should always reduce your load when you switch to a new, not-yet-tested primer type: “The primer is, in my experience, the greatest variable that can change the performance of a load combination, which is mostly to say ‘pressure’. Never (never ever) switch primer brands without backing off the propellant charge and proving to yourself how far to take it back up, or to even back it off more. I back off one full grain of propellant [when I] try a different primer brand.”
Priming Safety Tips by Zediker
1. Get a good primer “flip” tray for use in filling the feeding magazine tubes associated with some systems. Make double-damn sure each primer is fed right side up (or down, depending on your perspective). A common cause of unintentional detonation is attempting to overfill a stuffed feeding tube magazine, so count and watch your progress.
2. Don’t attempt to seat a high primer more deeply on a finished round. The pressure needed to overcome the inertia to re-initiate movement may be enough to detonate it.
3. Don’t punch out a live primer! That can result in an impressive fright. To kill a primer, squirt or spray a little light oil into its open end. That renders the compound inert.
4. Keep the priming tool cup clean. That’s the little piece that the primer sits down into. Any little shard of brass can become a firing pin! It’s happened!
Here is an important safety reminder for readers who employ hand priming tools with trays of multiple primers. In this instance a primer detonated while being seating in a pistol cartridge case. This caused a “flash-over” which instantly ignited multiple other primers in the tray. That, in turn, caused a small explosion which broke the tray and Hornady Handheld Priming Tool. The user, a Forum member, reported this last summer in our Shooters’ Forum.
Report by Forum Member about Primer Flash-Over Incident
I had a small scare the other day while reloading. My experience might be a helpful reminder to people to keep safe and follow proper procedures and not be sloppy.
A primer popped (went off) when being seated and (while hot) flashed over into the tray causing other primers in the tray to detonate. That, in turn, broke my Hornady Handheld Priming Tool and injured my hand.
Here is my loading area after the incident. Note the damage to the hand priming tool. I’m not not sure where the rest of the pieces are.
Injuries to User — Multiple Cuts on Hand and Forearm
This photo shows the resulting cuts to hand. Fortunately they seem superficial. Note — I was wearing latex gloves so my hand had some protection.
The blast also blew my glasses off my face and they landed about six feet away. Be careful folks and wear your eye protection! EDITOR: Guys — This is yet more compelling evidence why handloaders should always wear eye protection!
More Details — Primers Were Federal LP, Case Primer Pockets Had Been Checked
The forum member noted: “Just in case people are interested, I didn’t notice anything different from usual. Cases were Privi Partizan (PPU) factory brass on their fourth firing. The primers were Federal Large Pistol. The cases had been ultrasonic cleaned and the primer pockets checked prior to priming. I had primed around 50 cases during this session before the accident.”
Here is a close-up of “flashed” primers — note the empty but dented cups and triangular “anvils”:
Two other Forum members reported feeding issues with this particular Hornady handheld priming tool and one other member, Kiwi Greg, suffered a multi-primer detonation causing injury: “I had the same thing happen a few years ago now with the same Hornady hand primer with 50-75 [Federal 215M primers] in the tray. Made a bit of a mess of both my hands, blew primers and tray all around my large workshop. Scared the [heck] out of my employee who was near me at the time. Some stitches, bandages and it was all good, [but with] some wicked scars… I think a primer had turned side-on when being put into Bertram .408 CT brass which had nice tight pockets and required more force than usual. I use RCBS [priming tools] now with a flat bar that slides across to prevent sympathetic detonations. I hadn’t heard of it before, but found out it wasn’t that uncommon after it happened.”
Can you form a wildcat cartridge such as the 6 Dasher without expending primer, powders, and bullets? Absolutely. Using the hydro-forming method you can form improved cases in your workshop with no firing whatsoever, so there is no wear on your precious barrel. Watch this video to see how it’s done:
6 Dasher Case Hydro-Forming Demonstration:
Forum member Wes J. (aka P1ZombieKiller) has produced a helpful video showing how to form Dasher cases use the Hornady Hydraulic forming die kit. This includes a two-part die (body and piston), and a special shell holder. To form the case, you insert a primer in your virgin brass, top the case off with with a fluid (water or alcohol), then run the case up into the Hydro-forming die. A few stout whacks with a hammer and your case is 95% formed.
This same procedure can be accomplished with a Whidden Gunworks 6mm Dasher hydraulic form die. We like the Whidden hydro-forming die for its production quality and consistent results. This Whidden system works great according to our Forum members.
Hydro-Forming Procedure Step-by-Step:
1. Insert spent primer in new 6mmBR brass case.
2. Fill with water or alcohol (Wes prefers alcohol).
3. Wipe excess fluid off case.
4. Place case in special Hornady shell-holder (no primer hole).
5. Run case up into Hydraulic forming die.
6. Smack top piston of forming die 3-4 times with rubber mallet or dead-blow hammer.
7. Inspect case, re-fill and repeat if necessary.
8. Drain alcohol (or water) into container.
9. Remove primer (and save for re-use).
10. Blow-dry formed case. Inspect and measure formed case.
Wes achieves very uniform cartridge OALs with this method. He measured ten (10) hydro-formed 6 Dasher cases and got these results: two @ 1.536″; 2 @ 1.537″; and 6 @ 1.538″.
Three or Four Whacks Produces a 95%-Formed Case
With a Whidden or Hornady hydro-forming die, hydraulic pressure does the job of blowing out the shoulders of your improved case. The process is relatively simple. Place a spent primer in the bottom of a new piece of brass. Fill the case with water, and then slip it into a special Hornady shell-holder with no hole in the middle. Then you run the case up into the forming die. Now comes the fun part. You gently insert a plunger (hydraulic ram) from the top, and give it three or four stiff whacks with a mallet (or better yet, a dead-blow hammer). Remove the plunger and you have a 95% formed case, ready to load.
Special Shell-Holder
Hornady supplies a shell holder made specifically for the hydro die; there’s no hole in the bottom of it. Just insert a spent primer into the primer pocket and you’re ready to go. The spent primer, combined with the solid shell holder, keeps the water from seeping out of the primer pocket. The primer pushes out a little bit during this process, but it’s impossible for it to come out because of the way the shell holder is designed. The shell holder has a grove which allows the case to slide out of the shell holder even when the primer protrudes a bit.
The 6mm Creedmoor has become one of the most popular cartridges for PRS/NRL competition, and it also works great in the varmint fields. Good Lapua 6mm Creedmoor brass is available, and there are many 6mm Creedmoor factory rifle options now, such as the Savage Model 110 Elite Precision with MDT chassis and the Ruger Precision 6mm Creedmoor with 26″ barrel.
Do-It-Yourself 6mm Creedmoor Rifle Project — UltimateReloader.com
Another smart, cost-effective option for 6mm Creedmoor fans is to build your own rifle, starting with a Howa barreled action. The Howa 1500 is a mag-fed, flat-bottomed bolt action that ships with the excellent HACT 2-stage trigger. Howa 1500 full-size barreled actions have been available with a variety of barrel lengths and contours, starting at $429.99 at Brownells. Howa 1500 Mini barreled actions are available starting at $399.99 at Brownells.
There are quite a few good stocks/chassis systems now offered for Howa 1500 actions, including the excellent KRG Bravo Chassis, which features an ergonomic composite outer shell over a precision-machined inner chassis.
Gavin Gear fitted a KRG enclosed fore-end and Mystic Precision MPOD to his 6mm Creedmoor rifle.
Our friend Gavin Gear of UltimateReloader.com has built a nice 6mm Creedmoor rifle using a Howa 1500 heavy-barreled action and KRG Bravo Chassis. This project has proven very successful. The rifle has shown impressive accuracy and outstanding reliability. Gavin has used this rifle in some practical/tactical competitions, and it did well.
In this video, Gavin fits KRG’s enclosed fore-end to the KRG Bravo chassis. This accessory fore-end features a top Picatinny rail and various attachment options on the sides and lower section. Here you can see the enclosed fore-end (upper right) next to the factory forend included with the KRG Bravo (lower left):
Gavin then fits a Mystic Precision MPod from EGW. This stable, wide-base bipod uses a T-Slot rail for rifle attachment, and has legs that adjust independently for height. The video shows the rifle coming together step-by-step. This is something the average guy can do with simple tools — no gunsmithing is required, because the Howa barreled action is pre-chambered for the 6mm Creedmoor cartridge.
In the video above Gavin unboxes his Howa 1500 barreled action from Brownells. He then mounts the barreled action to the KRG Bravo stock, checking the torque levels. Next Gavin borescopes the hammer-forged barrel (5:55) noting: “What I saw I liked — there are practically no tooling marks. The finish on the lands and grooves looks really good”. Lastly, Gavin tested the trigger with his TriggerScan TS-11 (6:24), confirming a two-stage pull weight of about 2.25 pounds out of the box.
BONUS: PRB 6mm Creedmoor and 6.5 Creedmoor Load Survey
The Precision Rifle Blog (PRB) has compiled Load Data from PRS Competitors, for both 6mm Creedmoor and 6.5 Creedmoor. For new 6mm Creedmoor shooters, this PRB study is a good place to start. PRB surveyed the match loads for “173 of the top-ranked precision rifle shooters in the country”. One cautionary note: These PRS guys may be loading fairly hot, so work up gradually, 0.3 grains at a time. CLICK HERE.
Sierra Bullets has published load data for the 6mm Creedmoor cartridge. Sierra has released very comprehensive 6mm Creedmoor load data in printable PDF format, covering fifteen (15) different bullets from 55 to 110 grains. NOTE: Hornady brass was used for Sierra’s 6mm Creedmoor load tests, not the superior, stronger Lapua 6mm Creedmoor brass with small primer pockets. Hand-loaders using Lapua brass may have to adjust their loads.
Here are Sierra’s 6mm Creedmoor Load Data Charts for 90-95 grain bullets plus the 107gr MK. There are six more tables for other bullet types on Sierra’s 6mm Creedmoor Load Data Page.
Many shooters prefer to deprime their brass before resizing. That way they can tumble cases or keep primer debris off their main press. To deprime cases before sizing or cleaning you can use a Decapping Die. This pushes out the spent primer without changing the neck or body of a case. Such decapping dies work fine, but they do require the use of a press. Here is a handy alternative — a cool tool that allows you to deprime brass anywhere — no press needed.
Handheld Primer Removal Tool From Frankford Arsenal
This cleverly-designed Hand Deprimer Tool allows you to deprime cartridge cases without a press. This hand-tool from Frankford Arsenal will deprime brass and capture primers conveniently. You can deprime your cases while watching TV or relaxing in your favorite chair.
This device lets you remove spent primers anywhere — no press needed and all the mess (cups/anvils/residue) stays in the capture chamber. This tool comes works with nearly all common case types up to .338 Lapua Magnum. With good leverage, this tool does the job quickly and efficiently. Forum members have praised this handy tool, but recommend wearing a thick glove if doing more than 100 cases in a session.
This handy depriming tool is very versatile. With a universal, cylinder-style cartridge-holder, the tool can deprime a wide variety of cartridge types from .20 caliber up to .338 caliber. Three different plastic collets are provided to handle for different diameter cases. Spent primers are captured in a removable spent primer catch tube. Simply twist off the clear catch tube to dump the spent primers. With die-cast metal construction, this tool should last through many thousands of depriming cycles. MSRP is $54.99. Right now it is $47.98 on Amazon with 82% 5-star ratings.
User Modifications — Grip Padding and High-Volume Capture
Many users recommended putting some kind of padding on the grip and front lever to reduce pressure on the fingers. This can be done with a wrap or a rubber covering. In addition, users have adapted the unit with an attached hose and large primer capture jug. If you depriming hundreds of cases at a time, this hose conversion may make sense.
Review from actual owner: “I have owned one of these for about eight years and deprimed over 10,000 cases with this tool. I have never had to replace a single part on it. You will have to adjust the return spring every so often, but that is a very easy task. If you are depriming a LOT of brass, the handle can tear up your hand a bit. Solution: Wear a Mechanics Glove. Easy Peasy! You will NOT regret this purchase.”
How to Adapt Tool for Small Flash Hole Brass
The Frankford Arsenal Deprimer Tool was designed for LARGE flash-hole brass. To deprime brass with small (1.5mm/0.59″) small flash-holes, as found on Lapua .220 Russian and 6mmBR cases, you will need to reduce the pin diameter. Frankford Arsenal currently recommends purchasing a replacement pin and “sanding it down” to the smaller diameter. NOTE: This is NOT difficult — simply spin the stock pin in some sandpaper.
by Sierra Bullets Ballistic Technician Gary Prisendorf
Occasionally someone will ask, “Why did I get a flyer that didn’t go in with the rest of my group?” If I had an answer that would stop flyers from happening, I would be rich.
There are many reasons why this can happen. Everything from gripping a forearm differently to variations in the brass casing, the list goes on and on. Most of the time the flyer is usually shooter induced and sometimes what you may think is a flyer, is just part of your group. There are a lot of shooters, that go out and test a load and they may shoot a 3/8” group at 100 yards and think that load is good. But I have seen far too many times that you can shoot another group, same load, same rifle and the next time you may get a 1 ¼” group.
The total opposite can also occur. You may shoot a 1 ¼” group and turn around and follow it with a 1/2″ group without changing anything. If you only shot the one group, you might decide that load wasn’t any good and move on to something else without really knowing what that load was capable of.
To really determine how a particular load is performing we need to shoot multiple groups and take an average of the group sizes to really see what that rifle/load combination is really capable of.
I suggest shooting a minimum of three 5-shot groups and averaging the group sizes before deciding if the load is acceptable or not. Obviously the more rounds you shoot for a group and the more groups that you shoot, you will get a much better representation of what that particular combination can do.
Now I’m not saying to go out and shoot 30 groups with 50 rounds in each group to determine how well your load is shooting. That would be a bit pointless, in some cases it would be time to re-barrel your rifle before your load development was finished.
In most cases, I feel that three to five, 5-shot groups will give you a pretty good representation of how a load will perform in that specific firearm.
The Summer Solstice is June 21, 2023, just a week away. And July is coming very soon. That means “peak heat” summer conditions. It’s vitally important to keep your ammo at “normal” temps during the hot summer months. Even if you use “temp-insensitive” powders, studies suggest that pressures can still rise dramatically when the entire cartridge gets hot, possibly because of primer heating.
It’s smart to keep your loaded ammo in an insulated storage unit, possibly with a Blue Ice Cool Pak if you expect it to get quite hot. Don’t leave your ammo in the car or truck — temps can exceed 140° in a vehicle parked in the sun.
To learn more about how ambient temperature (and primer choice) affect pressures (and hence velocities) you should read the article Pressure Factors: How Temperature, Powder, and Primer Affect Pressure by Denton Bramwell. In that article, the author uses a pressure trace instrument to analyze how temperature affects ammo performance. Bramwell’s tests yielded some fascinating results.
For example, barrel temperature was a key factor: “Both barrel temperature and powder temperature are important variables, and they are not the same variable. If you fail to take barrel temperature into account while doing pressure testing, your test results will be very significantly affected. The effect of barrel temperature is around 204 PSI per F° for the Varget load. If you’re not controlling barrel temperature, you about as well might not bother controlling powder temperature, either. In the cases investigated, barrel temperature is a much stronger variable than powder temperature.”
This Editor had the personal experience of 6mmBR hand-loaded ammo that was allowed to sit in the hot sun for 45 minutes while steel targets were reset. The brass became quite warm to the touch, meaning the casings were well over 120° on the outside. When I then shot this ammo, the bullets impacted well high at 600 yards (compared to earlier in the day). Using a Magnetospeed, I then chron-tested the sun-heated ammo. The hot ammo’s velocity FPS had increased very significantly — all because I had left the ammo out in the hot sun uncovered for 3/4 of an hour.
LESSON: Keep your ammo cool! Keep loaded ammo in the shade, preferably under cover or in an insulated container. You can use a SEALED cool pack inside the container, but we do NOT recommend H20 ice packs. And don’t have the container do double duty for food and beverages.
Powder Heat Sensitivity Comparison Test
Our friend Cal Zant of the Precision Rifle Blog has published a fascinating comparison test of four powders: Hodgdon H4350, Hodgdon Varget, IMR 4451, and IMR 4166. The first two are Hodgdon Extreme powders, while the latter two are part of IMR’s Enduron line of propellants.
The testers measured the velocity of the powders over a wide temperature range, from 25° F to 140° F. Hodgdon H4350 proved to be the most temp stable of the four powders tested. [NOTE: New Alliant Reloder TS 15.5 has also proved very temp stable in AccurateShooter’s range tests.]
For some applications, we prefer a non-bushing FL die over a bushing die. Shown here are three Forster full-length sizing dies, with necks honed to three different dimensions: 0.265″, 0.266″, and 0.267″.
The Honed Full-Length Sizing Die Option
There are many good options in full-length (FL) sizing dies. Most precision hand-loaders prefer FL dies with neck bushings. These let you adjust the “grip” on your bullet by using larger or smaller bushings. FL bushing dies are available from Whidden Gunworks, Forster, Redding and other makers.
Conventional, non-bushing full-length sizing dies can create ultra-accurate ammo with very low run-out. But many conventional non-bushing FL dies have an undersized neck diameter so you end up with excess neck tension, and you work the brass excessively.
There is another effective option, one that promises extremely low run-out. The honed FL die is a full-length sizing die that has the necked honed to provide a precise fit to the case-neck. When done right, honed FL dies produce extremely straight ammo — as there are no issues with bushing alignment (or bushings that are not perfectly concentric). This Editor owns honed dies from Forster, Redding, and Whidden Gunworks. They all perform extremely well, delivering match ammunition with extremely low run-out measured with a 21st Century Concentricity Gauge.
In one of the most popular articles we’ve ever published, Bugholes from Bipod, California shooter “Froggy” explained why he prefers honed dies for his tactical ammo.
Q: Do you FL size every time? Do you use custom dies?
Absolutely, I full length resize all of my brass every time I reload. And guess what? I’ve never had a feeding problem.
I do use a modified sizing die, without bushings. My FL resizing die has been custom-honed in the neck area to give .0015″ press fit on the bullet. I also put a slightly larger radius at the neck shoulder junction. I feel that this helps to seal the chamber. With this die, I get consistent neck tension every time–without bushings. Bushings are useful when you’re fishing around for a good load. But once you find the right amount of sizing for ideal neck tension, you can do this better with a customized FL die.
6.5 Guys Review Forster Honed Full-length Dies
The 6.5 Guys have reviewed honed FL sizing dies from Forster, explaining the pros and cons of this type of reloading die. They explained that, if you load a wide variety of bullets from different manufacturers, you many want to stick with a Bushing FL die. However, if you have settled on a particular bullet and found the “ideal” neck tension, then a honed die may make sense.
In this Gear Update, the 6.5 Guys discuss a service offered by Forster Products to custom hone the neck diameter of its full-length sizing dies to the customer’s specifications (to the thousandth). Whidden Gunworks also offers custom-honed FL dies.
This article is Copyright 2023 AccurateShooter.com. Any publication on another website is unauthorized and obligates monetary compensation and statutory damages.
In this .308 Win test, 70° F ammo shot 96 FPS slower than ammo heated to 130.5° F. And the 130.5° ammunition was 145 fps faster than ammo taken right out of the freezer (at 25.5° F). That’s a huge difference…
Summer Solstice is just two weeks away, so most parts of the country will soon be encountering peak summer heat. Some ranges in the Western states have already recorded temperatures well over 100 degrees F during matches. When dealing with extreme summer heat, you should make a serious effort to keep your ammo at reasonable temperatures. When possible, keep ammo in a cooler in the shade.
Never leave boxes of ammo out in the hot sun. Even with powders advertised as “temp stable” you can see significant velocity increases when ambient temps reach 90 degrees and above. This article explains how temperature extremes (both hot and cold) can alter bullet velocities. The velocity differences between very cold ammo and very hot ammo can be very large, as this article explains.
EDITOR’s NOTE: The Sierra tester does not reveal the brand of powder tested here. Some powders are much more temp sensitive than others. Accordingly, you cannot extrapolate test results from one propellant to another. Nonetheless, it is interesting to see the actual recorded velocity shift with ammo temperature variations in a .308 Win.
Written by Sierra Chief Ballistician Tommy Todd This story originally appeared in theSierra Bullets Blog
A few weeks ago I was attending the Missouri State F-Class Match. This was a two-day event during the summer and temperatures were hot one day and hotter the next. I shot next to a gentleman who was relatively new to the sport. He was shooting a basically factory rifle and was enjoying himself with the exception that his scores were not as good as he hoped they would be and he was experiencing pressure issues with his ammunition. I noticed that he was having to force the bolt open on a couple of rounds. During a break, I visited with him and offered a couple of suggestions which helped his situation somewhat and he was able to finish the match without major issues.
He was shooting factory ammunition, which is normally loaded to upper levels of allowable pressures. While this ammunition showed no problems during “normal” testing, it was definitely showing issues during a 20-round string of fire in the temperatures we were competing in. My first suggestion was that he keep his ammunition out of the direct sun and shade it as much as possible. My second suggestion was to not close the bolt on a cartridge until he was ready to fire. He had his ammo in the direct sunlight and was chambering a round while waiting on the target to be pulled and scored which can take from a few seconds to almost a minute sometimes.
This time frame allowed the bullet and powder to absorb chamber [heat] and build pressure/velocity above normal conditions. Making my recommended changes lowered the pressures enough for the rifle and cartridge to function normally.
Testing Effects of Ammunition Temperature on Velocity and POI
After thinking about this situation, I decided to perform a test in the Sierra Bullets underground range to see what temperature changes will do to a rifle/cartridge combination. I acquired thirty consecutive .30 caliber 175 grain MatchKing bullets #2275 right off one of our bullet assembly presses and loaded them into .308 Winchester ammunition. I utilized an unnamed powder manufacturer’s product that is appropriate for the .308 Winchester cartridge. This load is not at the maximum for this cartridge, but it gives consistent velocities and accuracy for testing.
I took ten of the cartridges and placed them in a freezer to condition.
I set ten of them on my loading bench, and since it was cool and cloudy the day I performed this test I utilized a floodlight and stand to simulate ammunition being heated in the sun.
I kept track of the temperatures of the three ammunition samples with a non-contact laser thermometer.
The rifle was fired at room temperature (70 degrees) with all three sets of ammunition. I fired this test at 200 yards out of a return-to-battery machine rest. The aiming point was a leveled line drawn on a sheet of paper. I fired one group with the scope aimed at the line and then moved the aiming point across the paper from left to right for the subsequent groups.
NOTE that the velocity increased as the temperature of the ammunition did.
The ammunition from the freezer shot at 2451 fps.
The room temperature ammunition shot at 2500 fps.
The heated ammunition shot at 2596 fps.
The tune window of the particular rifle is fairly wide as is shown by the accuracy of the three pressure/velocity levels and good accuracy was achieved across the board. However, notice the point of impact shift with the third group? There is enough shift at 200 yards to cause a miss if you were shooting a target or animal at longer ranges. While the pressure and velocities changed this load was far enough from maximum that perceived over pressure issues such as flattened primer, ejector marks on the case head, or sticky extraction did not appear. If you load to maximum and then subject your ammunition to this test your results will probably be magnified in comparison.
This test showed that pressures, velocities, and point-of-impact can be affected by temperatures of your ammunition at the time of firing. It’s really not a bad idea to test in the conditions that you plan on utilizing the ammo/firearm in if at all possible. It wouldn’t be a bad idea to also test to see what condition changes do to your particular gun and ammunition combination so that you can make allowances as needed. Any personal testing along these lines should be done with caution as some powder and cartridge combination could become unsafe with relatively small changes in conditions.
Report by Forum Member about Primer Flash-Over Incident
Hydro-Forming Procedure Step-by-Step:
Special Shell-Holder
To learn more about how ambient temperature (and primer choice) affect pressures (and hence velocities) you should read the article 
