Hmmp. Might not be doing anything to it but shooting cast bullets and having a ball. It took me a week to get all the copper out from the first 56 shots (cleaned in-between every 3-5 shots with brush, patches, and copper solvent, apparently that didn't help much). I decided to try cast and see if it was worth fooling with. The only mould that even comes close to filling up the space is the MP .22 NATO bullet. I had a bunch cast up from ACWW plus about 1% tin, air cooled, which wasn't my alloy of choice for anything supersonic, but they were there so what the heck. I honed a sizing die out to .2255" and checked/lubed/sized a bunch. First batch was over 14 grains of RX-7. First three went into the same hole, then three more opened it up to 3/4" at 75 yards. I chronographed the load at around 2250 fps but the SD was wild and the second group showed it. I bumped the load to 15 grains and shot ten into 1.4", averaging around 2340 fps. That's still better than most of the jaxketed I've shot through it.
Since it looked like this might actually work out, I pulled up QL, matched the data I already had (upped the SSIP to 3K and that dialed it in), and played with powders that would get me over 2500 fps with minimal pressure. 20 grains of IMR 3031 looked good at 28K psi/2580 fps, but it only actually made 2452 average FPS for ten shots. That's a book starting load for the Sierra 63-grain SMK, by the way. The SD was MUCH better at 36 fps, and the second shot cut the first shot's hole even with a 60 fps cold-suppressor boost. Nine went into 3/4" and I rolled the rifle in the bipod for the last shot, dadgummit!!! for a 1-1/8" group at 75 yards.
The only problem so far with doing this is I have to seat the bullet so the entire lube groove and just a titch of the rear driving band extends past the case mouth. There's enough in the neck to be secure during handling, but the cartridges are too long to go in the magazine. I got zero lead in the barrel and almost none on the brake or suppressor from 30 rounds total.