I would normally recommend an LFN for both guns and believe it would be best for your rifle also. However, since neither of us have a clue whether it would feed, you better contact me by email and have me send sample bullets so you can test how they cycle through your action. LBTisAccuracy@localnet.com
300 gr bullets could very well be suitable if your gun has a faster twist than the ACP pistols. If twist isn't faster, they may not stabilize at subsonic speeds. However, if jacketed 300 gr bullets shoot accurately, cast of the same weight will also, as they will be considerably shorter.
Perhaps of interest, cast being driven at the same subsonic speed as jacketed will be far quieter, whether from a supressed gun or regular barrel. Also powder choice is very important in keeping the report minimized. The best powder I've used in that respect is Hodgen Universal, and it should get a 300 grain cast out at subsonic speeds with fairly gentle pressures.