For subsonic 4570 the best thing you can do is load a big bullet if your rifle can do it, a 600ish grain postel etc gives me the best luck. 4570 is a big case... too big for good subs really. The best fix ive found is making your own big cast bullets to eat up the case space. Also can get rifle set up for cut down 4570 cases... lol but that's a rabbit hole.
If the acme cast bullets are not playing nice with your barrel they can do all kinds of crazy things on their way to target. Keyholes can happen very easily.
Commercial cast bullets tend to be of very hard alloys that play havok on some types of rifling, they may not obturate to the barrel well in low pressure loads, and often they simply are not fat enough. Excess pressure for the bullet base can easily make keyholes too. Even the bullet lube they used may not do well at low velocities or vice versa.
All this can easily cause barrel leading and distort the bullet to make it do crazy things in flight you don't normally see in jacketed bullets.
Another thing to consider... your cases may need cast bullet prep! Generally brass sized for jacket bullets will swage cast bullets down too small for your barrel, resulting in poor shooting ammo. Good cast bullets in 4570 are often .459 or bigger depending on what your rifle can chamber. Thiccc is good if it can chamber. Noe and Lyman make great tools to prep your cases for cast bullets to work well.
For powders... Trail boss tin star and unique have been the easiest to work with here, though very hard to find lately. Titegroup may be a little fast/harsh for a plain base cast bullet, causing your keyholes.
I'm not sure what your twist is, but my Marlin shoots slow 405 cast bullet very well. I'd be surprised if yours wont since that is sorta THE 4570 bullet. Also you are not shooting them very slow to where I'd think twist will be an issue unless henry really messed up on specs for that barrel.
Also worth mentioning that, unless I missed it, you'll want to use coated cast bullets in the suppressor. They make a mess in there otherwise, and a huge pain if the can cannot be taken apart.
