Back Block- This is teh part at teh rear of teh gun, that teh bolt locks into, and the cocking rod, is pushed back by...it is connected via teh Pump Arm, to teh ram at teh front which is screwed into the...
Semi Block- So called because originally, teh gun was a pump, and the Semi Block was sold, to enable AutoCocking..hence teh name...its teh bit at teh front with teh ram, lpr and 3 way or noid attached.