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May 27th, 2016

Primer Pocket Plugs Help You Measure Case Capacity Accurately

H20 Case Capacity measurement tool plug

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 sells a great little $11.99 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 $11.99 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 twelve bucks over the long haul.

Product Find by EdLongrange. We welcome reader submissions.
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June 9th, 2009

CMP Article Explains Use of Scoring Gauges

Gary Anderson, Director of Civilian Marksmanship (DCM), has authored an excellent primer on the use of plug gauges to score targets. Anderson reviews the different types and sizes of gauges and explains the sometimes confusing rules for there use. CLICK HERE to read the whole article in the First Shot, the CMP Online Magazine.

Anderson writes: “A frequently asked question at CMP concerns what are the correct gauges to use for scoring different targets and where can they be obtained. This is not a simple question to answer. There are inward gauges and outward gauges for most 10 meter and 50 foot targets and when each must be used varies from target to target. There are actually three different smallbore or .22 caliber inward gauges. And in High Power rifle matches governed by NRA Rules, the gauge used depends upon the caliber of the rifle being fired at each target. Moreover, ISSF and USA Shooting Rules give legal gauge dimensions in millimeters and NRA Rules use inches, while National Three-Position Air Rifle Council gives both. With so many rulebook variations, it is not difficult to understand why questions about scoring gauges are commonplace.

Scoring Gauge
A scoring gauge is a precision metal instrument with a spindle sized to fit into the shot hole and a flange or “measuring diameter” turned to a precise size that is specified in a competition rulebook. The gauge, which is often called a “plug,” is inserted into a doubtful shot hole. The scorer then examines or “reads” the edge of the flange to determine whether the doubtful shot is “in” (receives the higher value) or “out” (receives the lower value). A magnifying glass is usually used to aid the scorer in accurately reading where the edge of the flange lies. Scoring gauges come with various types of handles that are used to gently hold the gauge while inserting it in a doubtful shot hole. Scoring gauges available in the USA vary in cost from $4.00 to $15.00.

Inward or Outward Gauges
A first step in sorting out this question requires an understanding of the difference between inward and outward gauges. An inward gauge is read on the inside or side of the gauge that is closest to the target center. An outward gauge is read on the outside or side of the gauge that is away from the target center.

Inward gauges give direct readings— does the inside edge of the flange touch or break the scoring ring in question? Outward gauges give indirect readings — does the outside edge of the flange remain inside the outer edge of a scoring ring that is usually two rings outside of the scoring ring value that is being evaluated? Some short-range targets and all targets used at distances of 25 yards or longer are scored with inward gauges. Outward gauges are used to score certain shots on 5-meter BB gun, 10-meter air rifle and pistol and some 50-foot smallbore rifle targets. Outward gauges typically are used on smaller rifle targets that have miniscule dots for 10-rings and scoring rings that are very close together because reading those gauges to the outside on larger scoring rings is more accurate.”

Official Gauge Specs and Sources
Anderson’s full article goes on to list the specific gauges (with dimensions) used for a variety of disciplines: 5-meter BB gun, 10-meter Air Rifle, 10-Meter Air Pistol, 50-Foot USAS 50 Smallbore Rifle, 50-Foot A-36 Smallbore Rifle, ISSF/USAS .22 Cal. Rifle & Pistol, NRA Smallbore Rifle & Pistol, CMP Rimfire Sporter (and Sporter rifle), CMP Games High Power Rifle, and CMP EIC and NRA Highpower Rifle. Anderson notes that NRA Smallbore Rifle and Pistol targets, as well as CMP Rimfire Sporter targets, must be scored with a .22 NRA Inward Gauge (.2225-.224”) for all rings. Anderson suggests using an Eagle Eye scoring device to detect and score double or multiple shot holes.


Text and photos © 2008 CMP, used with permission.

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