Since you already have the .44M rifle, there are some boutique makers of heavy (~400gr) subsonics you may want to look into. Your twist rate may not be suitable though.

If I were starting from zero and on a budget, I’d run and get a Ruger American Ranch in .450 Bushmaster plus a suitable can. Hornady makes a 395gr subsonic. The only caveat is you’ll likely need to step up from a .45ACP suppressor even though the bore diameter is the same. I contacted SiCo about putting an Osprey 45 on a .450BM and they said no way José!

.300 Blackout is one candidate for that.

If you are willing to invest in a big bore can like a Silencerco Hybrid 46M, you could go with a .458 SOCOM upper (I recommend having Tony Rumore at Tromix build it, as he is *the* .458 SOCOM guru). It will throw big subsonic pills with lots of energy: plenty enough to knock down whitetails.

But remember, shooting subsonics means that *any* caliber you shoot is going to have significant drop beyond 150 yards. .300 Blackout subsonics with 1050 fps muzzle velocity are going to have 8” of drop at 100 yards, and 52” at 200. .458 SOCOM subs of similar velocity have drops that are about the same, but are delivering twice the energy. Time of flight, gravity, and the laws of physics simply can’t be wished away.

Thus far I have been considering a certain .44 Special load (with a 240 grain Keith semi-wadcutter hardcast bullet) out of my .44 Magnum rifle with a suppressor. The problem with that platform is that the bullet loses velocity fast and I cannot realistically/ethically shoot beyond 50 yards. I would absolutely LOVE to have something that extends that range to 75 yards or even better 100 yards. That has me wondering if this new cartridge with a .338 caliber, 307 grain expanding bullet would fit the bill.

Split the difference with 350 Legend, use all the small frame AR parts with the exception of the barrel. It’s literally 380 long rifle, and then load it with whatever .355 projectile, as light or as heavy as you want on top of however much powder suites you. This is a solution to a problem no one asked for, like 8.6blk

If you want to go big, go big!