This shocking photo of destroyed AR-15 bits and pieces was posted on Facebook by William Walter, a firearms instructor. William said this was “The worst AR-15 blow-up I have ever seen. The case head literally atomized …you can see the brass residue on the parts. The bolt was split in two also.”
There’s nothing inherently wrong with AR-platform rifles (well, except that they run dirty). Over the past six decades Eugene Stoner’s semi-automatic design has proven its merits in military and civilian applications. You may not know, but the original AR from 1955 was a .30-caliber. The Armalite AR-10 shot the 7.62×51 cartridge. Later, at the request of the U.S. Military, Armalite created a smaller version that became the M16/AR15. The rest was history. Now there are millions of AR “black rifles” in the hands of soldiers and sportsmen.
With so many ARs in circulation, it’s no surprise that some get used by folks who don’t know how to hand-load or otherwise fail to follow safe gun practices. The AR is actually a pretty sturdy rifle, but when it’s fed bad ammo, or abused, bad things can happen. Very bad things… commonly known as Kabooms.
The American Shooting Journal (ASJ) has compiled a set of particularly extreme AR Kabooms. Compiling the “evidence” from various web sites, ASJ has published nine (9) of the most Unbelievable AR-15 Fails. Here are two of the worst “AR-15 Fails”. CLICK HERE to see them all.
Nothing but busted parts after this catastrophic Kaboom…
Notice the bolt is still stuck in the barrel extension … with the rest of the gun in pieces.
Another view of this sad blow-up…
These catastrophic AR failures are eye-openers, that’s for sure. ASJ cautions: “Any weapon can fail if given the wrong ammunition, faulty reloads or a plugged barrel. Always be aware of what can happen at anytime if you fail to follow common sense and gun safety rules.”
ASJ Sources: Photobucket, Armory Blog, Pinterest, Northeastern Arms, Eric Nestor
Are you seeing unpredictable changes in Point of Impact on your target? Think you may have a scope issue? Well maybe not — when was the last time you checked your BARREL?
Yes scopes do fail, and scope bases/rings do get loose. But sometimes problems with erratic POI shifts are caused by a LOOSE BARREL. This issue came up recently in our Shooter’s Forum. One member complained that his zero was shifting from day to day — by as much as two inches at 100 yards. He was convinced he had a scope problem, based on erratic POI:
“I think my scope loses 1 to 3 MOA per day. When I shot my rifle Monday it was dead on. On Tuesday it was 1″ low. Then on Wednesday it was 1 or 2″ lower. I don’t get it. — the elevation knob never touched. Scope will track and return to zero that day perfect. Yes EVERYTHING has been checked, nothing loose. What is the chance the erector tube spring has gone south? For the record this is a Vortex GE. Never had a bad scope, but this has me wondering”. — LB
On Forum member told LB to send the scope right back to the manufacturer. Two other members suggested mounting the scope on a different rifle to test. Good advice. That’s generally a smart strategy before you conclude a scope has gone bad…
Could Problem Be the Scope Base?
Two Forum members, ExPiper and Dickn52, suggested checking the scope base, recounting their past experiences with troublesome bases. This was intelligent — anyone with a POI problem should check all the optics attachments:
“Went crazy one day chasing my impacts on a 100-yard target. Shots would group fine for three then go nuts for 4-5. I cranked and un-cranked for about an hour. Then I reached up and the base wobbled on the rifle. Removed scope, tightened base screws and back in business.” — Dickn52
“Years ago I had a problem [where] shots were climbing with almost every shot. I was blaming the scope. However, when removing the scope I noticed that the 20 MOA base was cracked and getting wider with every shot. Needless to say I replaced the base and the problem was solved. — ExPiper
Eureka Moment — The problem was the BARREL, not the Scope
There were many helpful suggestions, but member PirateAmmo steered LB to the real problem — a loose BARREL: “We had a problem on a home-built AR-platform rifle once, barrel was loose a tad…”
Member Snert chimed in: “Yep — I had a PPC that suddenly went 19″ low. Picked up gun off bench by barrel and felt a wiggle. I tightened the barrel and the POI went 19 inches up”.
Problem Solved — Barrel Tightened up and POI Back to Normal
The gentleman with the POI problem took the advice of PirateAmmo and checked his barrel. BINGO! Low and behold, the barrel WAS loose.
LB posted: “Barrel loose by about 2%, checked it twice before and didn’t find it the first two times”.
After LB re-tightened his barrel, his rifle started shooting normally again. No more shooting low by 1-2 inches. Problem solved. The fix didn’t cost a penny and now LB doesn’t have to send a perfectly good optic back to the manufacturer.
Lesson learned? Check ALL the variables before you assume a scope has gone bad. Along with the barrel, also check your action screw tension, and of course the scope base and rings.
Ever shot a straight-pull (aka toggle-link) action rifle? We like these action types, which were developed for Biathlon competitors who needed to shoot fast, but were not allowed to use semi-auto actions. The biathlon-style toggle action features a lever on the side of the action. Pull the lever back with your index figure to open the bolt, then push forward with your finger to close the bolt*. It’s fast and efficient. With some practice, you can cycle the action in a couple of seconds — nearly as fast as a semi-auto.
The Volquartsen Summit .22 LR rifle is based on a concept originally developed by Primary Weapons Systems. Designed for both competition shooting as well as small game hunting, this unique rifle features a straight-pull, toggle-style action with a Ruger 10/22 profile. That means you can run 10/22 magazines, swap into stocks inletted for the 10/22, and even use after-market 10/22 barrels. The CNC-machined receiver features an integral 20 MOA Picatinny Rail. The Magpul stock adjusts for length of pull. Weight is 5.8 pounds, so this is easy to carry in the field.
The Summit features a lightweight carbon fiber-wrapped barrel, threaded 1/2×28 at the muzzle to be suppressor-ready. One of our favorite features on this little rifle is the trigger — which has a light, crisp 1.75-lb pull weight. Watch the video above to see the Summit .22 LR in action. The tester, 22 Plinkster, was impressed with the rifle’s ease of use and accuracy. The Summit delivered a 0.277″ group at 50 yards, shot from the bench.
Tech Analysis of PWS-designed Action as used in Summit
See the Summit at SHOT Show in Las Vegas
Volquartsen will have the new Summit straight-pull rifle on display, along with a wide variety of rimfire firearms, at its booth at SHOT Show in Las Vegas this week. You’ll find Volquartsen at booth #11129. This year, Volquartsen will unveil a new 17 WSM version of the straight-pull Summit rifle.
* Biathlon shooters cycle their Fortner toggle actions even faster, using their THUMB to close the bolt. This allows them to get the index finger on to the trigger blade faster. The whole process takes maybe a second — as fast as you can say “snick-snick”. To use the thumb effectively, you need a stock with a more vertical pistol grip. The Magpul stock on the Summit moves your thumb a bit too far back to use comfortably to close the lever without shifting the hand.
Many shooters these days clean their cartridge brass ultrasonically, or wet-tumble their cases with stainless media (above). Both methods get brass clean and shiny, inside and out. However, when those wet-cleaning processes are completed, you’re left with a pile of soaking wet brass. How do you dry your brass quickly and efficiently, without unsightly water spots? Read on for some great answers…
In our Shooters’ Forum, Forum Gold Member Terry asked: “How do you dry your brass after Ultrasonic cleaning?” In an interesting Reloading Forum Thread, many smart suggestions were posted. A dozen fellow members outlined a variety of effective case-drying procedures, which work equally well for both wet-tumbled brass and ultrasonically-cleaned cases. Here are the Top 10 brass-drying suggestions from our Forum members.
TOP TEN Ways to Dry Cartridge Brass After Wet Cleaning
1. Food Dehydrator — Shake the brass in towel to get the bulk of water off. Next leave in the food dehydrator for 45 minutes or until there are no signs of moisture inside the cases. — Lawrence97
2. Lyman 5-Level Case Dryer — Rinse off cleaning solution(s), then load brass by type into racks in Lyman Cyclone Case Dryer. This is easier to load/unload than food dehydrators and holds more cases.
3. Hot Water + Compressed Air — Rinse all your cases as a batch using scalding hot water from the kitchen sink. Hot water evaporates off of brass very very quickly. Then hit them with compressed air. Takes 10 minutes. Simple. — SG4247
4. Oven Dry in Pre-Heated Oven — After pre-heating to 200° or so, turn off oven and put brass inside on a tray. Most important! Tell your wife what you are doing so she doesn’t crank it up to 425 to heat pizza! — MClark
NOTE: Many other members suggested oven drying at 150-200°. We recommend turning OFF the oven so you don’t cook your brass if you forget to remove the cases.
5. Towel Dry then Warm with Heat Gun — Roll brass in a towel until no more water shakes out. Lay out on cardboard box top and blow off with Harbor Freight heat gun. $9.99 on coupon. Two minutes of heated air and about half hour of wait and they are good to go. This is with primers removed. — Shaggy357
6. Compressed Air, then Sun Dry Outside – I rinse the brass, then blow them out with compressed air. Then, dependent on the time of year, lay them on a towel in the sun. — HogPatrol
7. Dishwasher on Dry Cycle – In the winter, I drop my wet brass cases neck-down on the rack pegs in the dishwasher, then turn on the dry cycle. In the summer…well, I’m in Texas. They go to the porch for a bit. — Toolbreaker
8. Alcohol Rinse then Air or Oven Dry — Rinse in 90% Isopropyl alcohol and either let air dry or stick in 175° oven for half an hour. Alternatively, use a dehydrator. — Zipollini
9. Slow Air-Dry in Loading Blocks — I have a reloading block with holes drilled in it. I simply load the block up and let it air-dry in the cupboard for a couple of days. — JCS
10. Wipe with Towel Then Anneal Normally — This thread is stirring my OCD side. Seems complicated for just drying — my brass dries just fine when I anneal it. This entire process can’t take an hour per batch. When finished, the brass is cleaned, annealed, and ready to size. — CHLuke
Deprime, then tumble brass with stainless media, water, Lemishine, and dish detergent.
Shake them easily in a strainer to knock out most media then grab 4-5 pieces, shake them over the bucket for the last of the media then inside a towel.
Finally blow out the primer pockets and wipe with a towel, load in the Annealeez.
Turn a Rough Factory Rifle into an Accurate Hunting Rig
Kiwi Nathan Foster has produced a good video for hunters with “under-performing” Remington Model 700 rifles. In this video, Nathan helps a client turn a badly-behaving Rem 700 into a reliable tack-driver. A customer had sent Nathan this rifle to rectify stubborn copper fouling. After bedding the rifle, the customer discovered that the rifle produced terrible groups due to the stubborn bore.
Nathan told us: “This was a grand opportunity to study what can go wrong with the M700 rifle with regards to both do-it-yourself work and flaws within rifle production. To help structure the video, we used the chapters of our Accurizing Book as reference steps for the video. This footage also works in conjunction with our free Remington bedding tutorials on YouTube.
Those who have watched the full M700 Troubleshooting video say this is one of the most helpful videos yet released on problem-solving with a factory hunting rifle. This video is especially helpful for those just getting into the accuracy game, as it walks the viewer through the basics of rifle tuning, then proceeds to more advanced methods of improving a badly-behaving rifle.
This video focuses on the Remington M700 and Rem clones, such as the Bergara rifle. However the lessons and techniques in the video can apply to any type of bolt-action rifle suffering heavy copper fouling. The video features detailed footage of barrel break-in and barrel-lapping procedures. These procedures may be beneficial for rough factory barrels. IMPORTANT! AccurateShooter.com recommends different break-in and maintenance regimes for custom, hand-lapped premium barrels — be conservative with fine custom barrels. Our best custom barrels have all shot superbly with minimal break-in and zero use of abrasives during break-in.
Troubleshooting the Remington 700 Rifle with Nathan Foster
NOTE: This is a free 70-second trailer video. The FULL Remington Troubleshooting Video is 1 hour, 16 minutes long and can be streamed through Vimeo-on-Demand for $12.00. Access Full Video HERE.
Nathan Foster of Terminal Ballistics Research in New Zealand, is a expert hunter and highly-respected author of a series of hunting and long range shooting books. Nathan’s first book, The Practical Guide to Long Range Hunting Rifles, is a classic — one of the best treatises ever written on choosing and using a hunting rifle.
The Remington 700 is the most popular bolt-action rifle in America, according to Gunbroker.com sales figures for new and “previously-owned” rifles. So, chances are that you (or a family member) have a Rem 700 of some vintage sitting in the gunsafe. Click the link above for a PDF version of the Remington 700 Owner’s Manual (also covers models Seven, and 673).
This article is part of Sinclair Int’l Step-By-Step Reloading Series. Most of the products mentioned in this article are sold through Brownells.com.
by Roy Hill, Brownells/Sinclair Copywriter
Making your own precision handloads is a meticulous journey with many steps, many important matters to consider, and many sets of measurements to calculate. For those who pursue the perfect group, the highest score, the really long accurate shot, the rewards more than outweigh the effort. Choosing the right cases, deburring the flash holes, making the primer pockets uniform, trimming the cases, and lubricating them are all familiar – and critical – steps along the journey. And now that your brass preparation is complete, you are at last ready to start running the cases through your press and fill them with primers, powder, and bullets. The very first die the brass encounters is the sizing die. You insert the case, work the press’s lever to return the case to its correct pre-fired dimensions – and the journey continues.
There are three types of sizing dies to think about: neck, full-length, and small base. All three have specific benefits and potential drawbacks, and you should choose the type of die you use by thinking very carefully about what kind of shooting you plan to do with your handloads. No matter which type you select, most sizing dies will also punch out the old spent primer with some sort of decapper assembly that uses a hardened steel rod. Many types of sizing dies use an expander ball inside the die to make sure the neck of the case will accommodate a bullet after being sized. With some size dies, the expanders are easily removable and interchangeable, letting you get exactly the neck tension you want. If you are reloading for pistol cartridges, carbide sizing dies allow you to quickly resize without applying any lube to the case. But rifle cases always need lube.
Neck-Sizing Dies Neck-sizing dies resize only the neck of the case. The benefit of sizing only the neck is that the brass is “worked” very little, letting you reuse the same cases many times over. Also, cases that have already been fired in your rifle are perfectly fireformed to fit that rifle’s chamber, which can help accuracy. However, neck-sized cases will fit only the specific rifle they were originally fired in, and may still require a little extra force to chamber or extract.
Sinclair recommends that neck-sized-only cartridges should not be used any in other rifle besides the one they were originally fired from [unless they are also FL-sized], or in any action other than a bolt-action. Neck-sized-only rounds are great for the target range or the benchrest but should not be used in critical situations like military or police operations, or hunting. And if you fire them enough times, neck-sized cases will still need to be full-length sized periodically for you to keep using them.
Full-Length Sizing Dies Full-length sizing dies do exactly what their name says: resize the full length of the case, not just the neck. Full-length sizing helps create handloads that will function in any rifle, not just the one from which the cases were originally fired. The potential downside of full-length sizing is that it may shorten case life because it works the brass more than neck sizing. But it’s possible to “tune” today’s full-length sizing dies so they barely work the brass at all, as this article by Sinclair Reloading Tech Ron Dague shows.
Illustration Shows How a Full-Length Sizing Die Works
Another way to reap the benefits of full-length sizing is to use Redding’s full-length bushing dies, which size the full length of the case but use a system of interchangeable bushings that enable you give the case neck the bare minimum of resizing needed. To see how finely adjustable bushing dies are, and how they resize the case while fully supported, CLICK HERE for Video. The neck bushing helps you precisely control the neck tension to help increase the consistency and accuracy of your handloads.
Small Base Dies
A Small Base Die is just another type of full-length sizing die, but one that is typically used when reloading for semi-automatic rifles, like the AR-15, M14, or AR-style .308 rifles. (It may also work well for bolt guns that need extra sizing on the lower section of the case.) A small base die works exactly like a full-length sizing die, only it compresses the brass just a bit more, usually about .001″ more, and may even push the case shoulder back just a hair. Small base dies give that extra bit of compression to the brass to help make sure the case will properly extract from a semi-automatic firearm. The upside is that you get precision handloads that should work flawlessly in your semi-automatic. The downside is case life is really shortened, especially compared to brass used only in one bolt-action rifle, because the brass is worked more.
Shoulder Bump Gauges
A handy tool for setting up your full-length sizing dies as close as possible to your rifle’s chamber is the Sinclair bump gauge. The bump gauge lets you resize the case as little as possible, to extend case life and help your handloads fit your rifle almost like a neck-sized only die. You use deprimed cases fired in your rifle and bump gauge inserts to help you set up the die so it resizes the case only about .001″ to .004″, depending on what type of rifle you’re shooting.
Video shows how to use a shoulder bump gauge to set up your full-length dies
Article Find by EdLongrange. We welcome reader submissions
Scary stuff — AK-74 after firing corrosive ammo and not being cleaned for a week.
Image courtesy ADCOFirearms.com.
No doubt you’ve heard the term “corrosive” used with respect to ammunition. But what exactly is “corrosive ammunition” (and how does it different from non-corrosive ammo)? What is the chemistry that leads to corrosion, and what cleaning procedures should you follow if you shoot corrosive ammunition? Brownells has come up with answers to these and other questions in a helpful TECH TIP video about corrosive ammo.
In this informative video, Brownells gun tech Steve Ostrem explains the primer-related chemistry that makes some ammo corrosive. The video then reviews suggested cleaning procedures you should follow after you have fired corrosive ammo through any firearms.
What Is “Corrosive” Ammunition?
What makes ammo “corrosive”? Generally speaking, primers are the problem. When corrosive ammunition is fired, the ignited primers leave a residue of corrosive salts. Typically these primers contain potassium chlorate, or sodium petrochlorate which, when burned, change into potassium chloride or sodium chloride. Sodium chloride is also known as common table salt.
Potassium chloride and sodium chloride are both very hygroscopic (i.e. they attract water). Because of that, these alkalis are rust generators. When exposed to the hydrogen and oxygen in the air (and moisture) potassium chloride and sodium chloride can form an acid that quickly causes metal rifle parts to rust and pit.
Given a choice, you may wish to avoid corrosive ammo altogether. However, for some types of fire-arms, particularly older military-style rifles, the most affordable ammunition may be corrosive. If you choose to use corrosive ammo, it is important to clean the gun thoroughly after use. After firing, you want to use an element that will neutralize the primer salts. Brownells suggests a water soak (see video above). Alternatively, Windex with ammonia can help neutralize the salts, but that doesn’t finish the job. After the salts have been neutralized and flushed away, basic anti-corrosion protectant (such as Eezox or other gun oil) should be applied to all metal parts.
This video ahows the effects of Corrosive Ammo after one month without cleaning:
To learn more about the proper procedures for cleaning rifles exposed to corrosive ammo, we suggest an article by Paul Markel on Ammoland.com. Markel, host of the popular Student of the Gun TV series, states that: “Windex (with ammonia) is the Corrosive Ammo shooter’s best friend. After you are done shooting your corrosive ammunition for the day, squirt the window cleaner liberally from the chamber down the barrel. Pull the bolt / bolt carrier / op rod if there is one and douse them as well. A couple of old cotton t-shirts will come in handy. A cotton barrel swab is a nice accessory but you can make do with patches. Some folks will rinse all of the ammonia and loosened corrosive salts off with hot water. Others prefer to wipe it all down and let the ammonia evaporate. Either way, once the corrosive salts have been tackled with the window cleaner, it is time for an all-purpose brush (old toothbrush) and some gun oil.” READ Full Article by Paul Markel.
Video Tip from EdLongrange. We welcome reader submissions.
Apparently reducing static charges on and around electronic scales can reduce their propensity to drift, lessening the problem of “wandering zero”. Just how and why static charges interfere with scale performance is unclear, but many shooters have noticed that static electricity can cause electronic scales to behave strangely. So how do you reduce static charges around your digital balance? GS Arizona, creator of the Rifleman’s Journal Blog, has found a very simple solution — an anti-static aerosol spray — that, by all indications, actually works. When this “spray-can solution” was suggested to GS by a fellow shooter, GS was skeptical. However, he tried the stuff and he says that it really does help the scale maintain zero over time, with much less observed drift.
Static Guard Reduced Scale Drift
GS Arizona explains that the use of “Static Guard” spray helped mitigate the problem of a drifting zero on his Ohaus Navigator electronic scale. He writes: “My electronic scale… suffers from drifting zero (as they all seem to). I’ve read dozens of forum posts about drift and how to minimize its occurrence, so I know this problem isn’t limited to my scale or my workshop. Sometime last year, John Lowther mentioned the use of anti-static spray as a solution to the drift problem. John stated that the spray had virtually eliminated drift for him.”
GS Arizona found that the Static Guard actually worked: “The spray works great, just as John said it would. I spray all surfaces that I touch with my hands and arms as well as the pan (top and bottom), the metal tray on which the pan rests and the table under the scale. In six months or so of using the spray I’ve re-applied it about two or three times; it certainly isn’t something that you need to do each time you sit down to load. Before using the spray, it was not uncommon for me to re-zero the scale 10 times in the course of loading 72 rounds; now it might need it once during a session.”
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. 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 illustration shows headspace measurement for the popular .308 Winchester cartridge, which headspaces on the shoulder. Image copyright 2015 Ultimate Reloader.
In this Brownells Tech Tip, Brownells gun tech Steve Ostrem explains what headspace is and why it’s one of the most critical measurements for nearly all firearms. Even if you’re an experienced rifle shooter, it’s worth watching this video to refresh your understanding of headspace measurements, and the correct use of “GO” and “NO-GO” gauges.
Headspace Definition
In firearms, headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum reference) to the face of the bolt. Used as a verb, headspace refers to the interference created between this part of the chamber and the feature of the cartridge that achieves the correct positioning. Different cartridges have their datum lines in different positions in relation to the cartridge. For example, 5.56x45mm NATO ammunition headspaces off the shoulder of the cartridge, whereas .303 British headspaces off the forward rim of the cartridge.
If the headspace is too short, ammunition that is in specification may not chamber correctly. If headspace is too large, the ammunition may not fit as intended or designed and the cartridge case may rupture, possibly damaging the firearm and injuring the shooter. (Source: Wikipedia)
Headspace Gauges
Headspace is measured with a set of two headspace gauges: a “Go” gauge, and a “No-Go” gauge. Headspace gauges resemble the cartridges for the chambers they are designed to headspace, and are typically made of heat-treated tool steel. Both a “Go” and a “No-Go” gauge are required for a gunsmith to headspace a firearm properly. A third gauge, the “Field” gauge, is used (as the name implies) in the field to indicate the absolute maximum safe headspace. This gauge is used because, over time, the bolt and receiver will wear, the bolt and lugs compress, and the receiver may stretch, all causing the headspace to gradually increase from the “factory specs” measured by the “Go” and “No-Go” gauges. A bolt that closes on “No-Go” but not on “Field” is close to being unsafe to fire, and may malfunction on cartridges that are slightly out of spec. (Source: Wikipedia)
To learn more, read Brownell’s longer article Headspace Gauges and How to Use Them. Among other things, this explains the relative lengths of “Go”, “No-Go”, and “Field” gauges. The “Field” is actually the longest: “The GO gauge corresponds to the SAAMI (Sporting Arms & Ammunition Manufacturer’s Institute) minimum chamber length, while the FIELD gauge usually matches the maximum chamber depth, or slightly less. NO-GO gauges are an intermediate length between minimum and maximum, that, technically, is a voluntary dimension. A firearm that closes on a NO-GO gauge and does not close on a FIELD gauge may not give good accuracy and may have very short cartridge case life from the ammunition re-loader’s standpoint.”