I have been meaning to get to a build of a large caliber subsonic rifle. Probably going to have to stay with bolt action for this. Reason I ask is I do have a Ruger 44 carbine that uses the 44 mag cartridge and I might be able to down load them to subsonic but then might have to mod the action since it is a semi-auto. I would also prefer to have more rounds available. You can get a pretty damn heavy bullet in a 44. I was thinking about something like the .308 as well. Probably will shoot this one a bit so round costs are a consideration. I like the .338 but that is probably way to expensive for what I am doing. This gun will have an effective range of 200yds. That is probably being the ability of a sub sonic 44 slug. They just are not built for that IMO. This gun will be a quiet giant and I really want to get some decent range out of this one. If I could get accuracy at 200, I would be happy. I am very happy with my sub sonic 22 at 100yds but I need to weight to comp the lack of ambition.