Board Thread:General Nerf discussion/@comment-27985798-20200527012131/@comment-27306930-20200626000741

Yes, a simple way is to use a small piece of plumbers abrasive cloth or mesh and bevel the inside edge of the brass tube by simply sanding it down by hand. In fact you can dull the outside edge as well if it's going to be exposed. But that probably won't be an issue with a shell. Here's a crude not-to-scale drawing.

Alternatively, with soft metals like brass you can use a countersink bit, put it in the end, and just hand turn it to bevel the inside edge. It gives you a nice precise angle, if that's important to you.