You can load anything to subsonic velocities. The 180's and 200's would be a natural fit to the application though.
You could get an AR-10 style in .308, or you could use a cheaper bolt gun.
The only issue for spooking them might be the response of the one you just shot.
Even a supersonic full power load will be have minimal report they could hear if you're shooting them from 100 yards away with a rifle.
For an AR-15/10 or rifle, you'd want some type of rifle optic. Check out this article for some applications
| Fighting Carbine Optics
The possibilities are just about endless. If you want to shoot them with a rifle, really pick the rifle and cartridge you want to kill them with and then get an appropriate suppressor for it.