I think it's safe to say that it's impossible for Hollywood to exactly replicate firearms physics. Actors cannot "act" felt recoil, mostly because recoil is a production of an item being held, not of one's body. You can try to fake your body's reaction to recoil, but it always comes off as looking fake.
Yes, the M4 used by The Governor to kill the Guardsman was missing its rear sight. There's a clear shot of it in the Season 3 link for the show at imfdb.net.
The Russian HMGs at the CDC were intended to be standins for M2s, since Hollywood armorys tend to have a tough time getting actual M2s