A subsonic 200gr bullet out of a rifle is very close to the velocity and energy of .45 acp round. Yes, it will do the work on a deer sized animal at 75yds with proper bullet placement. That said, the choice of a FMJ bullet for hunting is not in good form. While there have been a ton of deer killed with a FMJ, most of them are cruising along at twice the speed and it's illegal. You are going to have a little hole and maybe no exit wound and this could result in a deer traveling a long, long, long way before dying.
Now the specs say 200g Sierra match king which are hollow points, but at the bottom it says FMJ. My guess is that these are really hollow points if they are really SMKs. Lots of deer have been killed with SMKs, but most of them are running 2x+ the speed on impact and even then, expansion isn't predictable. At subsonic speeds, I doubt they will expand at all.
Find some subsonic hunting loads or make your own.
And yes, if your AR doesn't cycle, you can manually clear it. Just be on the lookout for unstabilized loads (key holing) and squibs and definitely re-zero your rifle. If your rifle is a 1:12 twist, it may not like a heavy, slow moving bullet.
There is a 2 minute drop adjustment (about 2" at 100 yds) on my gun going from 50 to 75 yds and another 2 min drop at 100 yds using 220g bullets. These rounds drop like a rock.
