Here’s a simple and low-cost method to accurately measure the headspace of your 6BR,.243 Win or.308 win brass when you “bump back” the shoulder. This tip is credited to Boyd Allen. You’ll need a.45 ACP (.40 SW will also work) case with the primer removed. The pistol case must be rounded and square-trimmed. We recommend that you size, trim and chamfer the case before chamfering. Then, slip the.45 ACP or.40SW case over the neck of an unsized, fired rifle case. Remove the primer. Align the cases between your calipers jaws and measure the distance from rim-to-rim.
Now you have the length of a rifle case that has been fired, but before sizing. Then, take a full length sized rifle case without primer and do the same, placing the pistol over the neck of FL-sized case. (Bottom photo). The difference between these two numbers is how much “bump” you are applying to your shoulder. Here, the difference is.0015″. The amount of bump that you need depends on your chamber and load, but.0015 to.002” would be a good starting point. Using this simple tool will help you avoid bumping your shoulder too much. This tool will also allow you to set the depth of the full-length die so that you get the right amount of bump every time.
Other Pistol Brass Types Also Work: Some people have used this method, but prefer to use 10mm or 40 SW brass instead of a.45ACP because the slightly smaller diameter pistol cases may fit their rifle cases better. It works just fine. Use whatever pistol brass case is best for your rifle brass. The method works equally well with 10mm or 40 SW brass. Make sure that the pistol brass is sized, trimmed chamfered and de-burred.
