July 13th, 2013

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.

H20 Case Capacity measurement tool plug

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).

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