Cheap Tricks: How to Measure Shoulder Bump Using .45 ACP Case
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, with primer removed. Make sure the .45 acp case is trimmed square and that it is round. We recommend you first run it through an expander, then size it, trim it and chamfer. Next, take the .45 acp 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 .45 acp 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.
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Tags: .45 acp, Boyd Allen, Bump Gauge, Headspace Measurement, Headspace measurements, Shoulder Bump
Yes it was my tip, and it is fun being famous
but I chose to use a case that lands closer to the middle of the shoulder such as a .38 special. The .45 variation was the site owner/editor/chief writer’s idea, one that I have not tried. Sometimes dies modify the radius at the point of the shoulder so that the change there, from sizing, may not correspond exactly to that which takes place at the datum line. The thing about this that I like is that it gives a new reloader a tool with which to measure an important dimension for almost no cost.
Dude do you realize how many peoples been doing that for years. I use a 40 Smith & Wesson case for my 6 mm Creed’s and 6.5 Creed
Like Brian above, I use a 10mm fired case to measure my 308s instead of a 45ACP because 10mm/40S&W are closer to the specified datum diameter. In fact my fire formed 308s measure 1.634″ with this case, which I believe is bang on the cartridge spec.
Best wishes from Chester, UK
Discovered the 45 ACP hack when loading 44-40, was a bit surprised to see it as a published hack.
BTW, it works well.