I use them for centering rings, BT-20 couplers (with a little shimming) and engine blocks, as well as cases for my removable baffles.
I can never get enough spent engine casings and actually have to ration them. Part of this reason is because I lost so many spent casings from poor drill-sanding techniques when sanding out the insides for removable baffle cases (I have since improved this and lose far fewer casings).