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Let us say each bullet takes about 1/20 th of a second to come out of the barrel of the gun.  Each bullet then accelerates from rest to a speed of 800 meters in a time duration of 1/20 = 0.05 sec.

Acceleration of a bullet = a = (800 - 0 ) / 0.05 sec = 16000 m /sec²
Force necessary to give the bullet this acceleration = m a
           = (150 /1000) kg * 16000 m/sec²
           = 2400 Newtons

The bullets exert the same amount of force on the machine gun as the machine gun exerted on the bullets.

Force necessary to hold the gun = recoil force of gun = 2400 Newtons