New Primer Pocket Plugs for H20 Case Capacity Measurements
When developing loads, it is important to know the true internal capacity of your cases, both fired and “as FL-sized”. In particular, when using the QuickLOAD program, it is vital to determine true case capacity. The default case capacity values listed by QuickLOAD may be off half a grain (or more) because brass from different manufacturers can vary considerably in capacity. Case capacity is a very important variable that will affect the pressure of a load and the velocity of your bullets.
To determine the true internal capacity of your cases, first weigh an empty cartridge case, then fill the case with water (all the way to the top of the neck) and weigh the case again. The difference in weight is your H20 capacity in grains. But how do you keep the H20 from flowing out the bottom? When measuring fired, unsized cases, you can simply leave the spent primer in the pocket. However, if you want to measure new brass or “as-FL sized” cases that have been deprimed, you’d have to insert a spent primer to “stem the tide”. Until now that is… 21st Century Shooting has come up with a simple tool that plugs the bottom of the case so you can measure H20 capacity with ease.
When we saw 21st Century’s Primer Pocket Plug we thought “That’s smart — why didn’t someone think about that a long time ago?”. This handy “end-cap” lets you quickly measure multiple new brass cases or deprimed FL-sized cases so you can get an average H20 capacity. The primer pocket plugs are NOT case-specific (they feature an O-ring that fits the pocket). One version will work with all small-primer-pocket cases, while another works with all large-primer-pocket cartridge types. Price is $19.95 for either small-pocket or large-pocket version.
NOTE: If you want to measure H20 capacity on fired, sized brass, but don’t want to shell out the money for the tool (or re-insert a spent primer), here’s a simple suggestion. When you size your case, first remove the decapping rod from the die. Then you can FL-size the case without removing the primer. Of course, you will eventually have to knock the primer out, and that requires putting the decapping rod back in the die and running the case through a second time. To avoid that hassle, the Primer Pocket Plug may be worth the $19.95 over the long haul. We just ordered one of each (small and large).
Product Find by EdLongrange. We welcome reader submissions.Similar Posts:
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Tags: 21st Century, Case Capacity, H20 Measurement, Primer Pocket, Reloading
I got caps sitting all over my floor, under my press’s. Why not just seat a spent primer in a new case, and knock it out when your done?
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Or you can size cases with the decapping rod out, fill w/H20 and weigh them, then use a universal decapping die to pop the spent primer without having to run it thru a sizing die again.
Bob: You are correct. However, most of the universal decapping dies have a 0.0625″ or larger diameter decapping pin that will stick in the 0.059″ flashholes of 220 Russian, 6PPC, 6mmBR, 6.5×47 cases etc. — unless you turn down the diameter of the pin. For that reason, we don’t routinely tell people to use a universal decapping die (without qualification). But if one has large flash holes, the procedure you recommend will work fine.
When filling with water should the water level be even with the top? Or should the water level be allowed to stay just above the top where the surface tension of the water holds it just above the case.
To get a really accurate measurement of load density vs. case capacity, you should insert the bullet you’re planning to use, weigh the case with the primer pocket sealed, remove the bullet, fill with water, re-insert the bullet and weigh again. The difference in weight will give you a much closer approximation of load density, if that’s what you’re after. Otherwise, I’m not sure how useful “case capacity” is. Can someone please enlighten me?
Using the right program H20 capacity can give you spot on velocities. My buddy ran some data based on his .223 useing varget.
We then cronographed my 308 load. Based on his live data he was able to tell me what my case specs where. I went home and confirmed everything he said.
So why is it useful, If you decide to switch bullets, change seating depth, neck length, or even get a batch of new brass, you name it, you will know what charge to use to match your last Cherry load.
Also, you will know what cases need to be removed from the lot. 2 grains of H2O could effect your velocity enought to keep you out of the XRing.
Yes loading a bullet would give accurate measurement but you just turned your case into a hydrolic piston, could blow your plug out , expand a case, rust your tools, it would not be very efficient. A loading program takes bullet specs into consideration.
I do have to laught, the chewing gum just gave me an idea, I just saw my kids silly putty sitting one the table.
This may not be a tool you use often but seems quick and can be used on freshly tumbled brass with primer removed.
I just typed this long winded answer and it didn’t post. Why is it useful? I didn’t think so until my buddy ran my velocity and was able to tell me what my case specs where useing quick load.
So I’m sold on H20 it provide very accurate load data when you use new brass, change bullets ect.
My buddy was able to run my velocity tell me my current case specs and also let me know how much variation in cases I could get away with before it would effect my scores.p and drop me out of the XRing ect.
At 100 yards it doesn’t mean much but at 1000 I want the tightest vertical I can get so I can concentrate on wind.
DrBob,
Two specifications:
Case water capacity
Case water overflow capacity.
The former is what you describe measuring. It’s limitation is it is only accurate for one bullet seating depth.
Case water overflow capacity is not literally an overflow, but you take a case trimmed to nominal trim length and fill it with water level with the mouth (no meniscus). Any program (like QuickLOAD) that knows the size and shape of the bullet can then subtract the bullet volume that is seated into the case for any seating depth you want to try.
In general, if the round peaks at a pressure in excess of 30,000 psi, you measure the case water overflow capacity of the fired case because that’s the volume the powder will be burning in for most of the way to the pressure peak. If the round peaks below that level, then the case water overflow capacity of the case as-resized gives a better pressure and velocity prediction.