The weapon is a very old anti-tank gun from the thirties by the looks, When he inserts the shell the breech should close atuomatically sealing the shell in, except it doednt work properly. On the second attempt he attempts to speed up and force it in. With an old, worn mechanisim the breech closes and the firing mechanisim goes and sets the shell off.
The gun has quite a bit of recoil so the breech hits him in the lower chest/stomach. Probably has some very interesting bruising if not broken ribs or internal injuries.
I think its real, guns that old often worn and have little problems if not properly maintained.
Bet that hurt!