This jig and a chop saw from HF make quick work of converting brass to 300blk. I use a Forester case trimmer to make the final trim after sizing.. cut a few rounds with the chop saw, just behind (but close to) the shoulder, size and trim them and see how much you have to remove in the final trim. Adjust your jig for cutoff to minimize the final trimming and it is pretty damned quick and easy. I have converted a LOT of brass this way with no issues, investment was about $30 iirc, as I already had the chop saw for arrow cutting years ago.
That is the jig I use and it works like a champ. Indexes and pops the cut round out when you stick the next one in. Just add a chop saw and yes, you should have some ID bands for your mags (I put one on my weapon too, at the rear of the handguard and also have 300BLK on the ejection port cover to ensure I don't mix anything up in a moment of haste.. LOL)
