My understanding of the "retina" situation is that basically, because of the proximity of a RIFLE shooter to the concussion of the round being fired (ie cheek against the stock) much more of the shock/impulse of the round is transferred to the shooter, as compared to firing an M2, where the shooter sits/stands behind the weapon, and fires it with the spade grip, where the contact between the shooter and the weapon is limited to the hands.
I've been told that the .50 rifle shooters in the CF have a daily limitation placed on the number of rounds they are allowed to fire to limit possible damage to themselves. (50 rds)
Take it with a grain of salt if you wish, but the concussion transmitted directly into your cheek with a good cheekweld will be MUCH more damaging to you than the simple blast concussion from standing near or even firing a mounted .50 cal from arms length.
NavyShooter