Forming .300 Blackout Brass
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Step 1 - Case Selection and Inspection
Don't dive into your brass bucket and start randomly trimming those range pick-ups just yet. If you're going to make your own .300 Blackout cases, the initial parent case selection is very important.
Figure 1 - .300 AAC Blackout Chamber Showing Diameter at the Neck
You can save yourself quite a bit of time and effort if you use only cases with the proper wall thickness. Take a look at the chamber drawing in Figure 1. Notice that the diameter of the chamber at the location of the case neck is 0.3350". Since the bullet has a maximum diameter of 0.309", that leaves only 0.026" of total space available for the brass. There should be about 0.002" of total clearance around the neck for reliable feeding, so the maximum case wall thickness is about 0.012". Keep in mind that 0.012" leaves almost no room for irregularities in case concentricity, so in reality you're better off with cases that are slightly thinner than this; a 0.0110" to 0.0115" wall thickness is just about perfect.
Click on the image below to make very large, and look at the case wall thickness minimums.
Color a few of the problem cases with a black felt tip marker and see where it has worn off after chambering the cases. If they are rubbing on the case body or the shoulder and base of the case the cases may need to be sized with a small base die and have the shoulder bumped back further. If only the necks have rub marks the the necks are too thick.