January 6th, 2022

Cheap Tricks: Measure Shoulder Bump Using a Pistol Case

.45 acp pistol case bump gauge headspace tool

Here is a simple, low-cost way to get reliable readings of case headspace when you “bump” the shoulder back on your 6BR, .243 Win, or .308 Win brass. Credit Boyd Allen for this tip. First, you’ll need one .45 acp case (.40 SW works too), with primer removed. Make sure the pistol case is trimmed square and that it is round. We recommend you first size it, trim it and chamfer. Next, take the .45 acp (or .40 SW) case and slip it over the neck of a fired, unsized rifle case with the primer removed. Align the two cases between the jaws of your calipers and note the length from rim to rim (See photo below, with striped case).

OK, now you have the length for a fired rifle case BEFORE sizing. Next, take a full-length sized rifle case (without primer) and do the same thing, placing the pistol case over the neck of the FL-sized case (Bottom Photo). The difference between the two numbers is the amount of “bump” or set-back you are applying to the shoulder. Here the difference is .0015″. The amount of bump you need varies with your chamber and your load, but .0015-.002″ is a good initial setting. By using this simple tool, you can avoid bumping the shoulder too much. This will also help you set-up the depth of your full-length die to get the proper amount of bump each time.

Other Pistol Brass Types Work Too: Some folks have used this method but they prefer to work with 10mm or .40 SW brass rather than a .45 acp, because slightly smaller-diameter pistol cases may conform to the shoulders of their sized rifle cases a bit better. That works fine — use whatever pistol brass case works best for your rifle brass. We got very repeatable results with .45 acp brass but the method also works with 10mm or .40 SW brass. Just be sure the pistol brass has been sized, trimmed, chamfered, and de-burred.


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