Answer;
= 312 Newtons
Explanation;
The bullet has a mass of 0.005 kg, and a velocity of 320 m/s, so we need to find it's final kinetic energy. Â
KE = 1/2*m*v^2
   = 1/2*0.005*320^2
   = 256 Joules. Â
Divide this by the distance over which this energy was received and you have the force that provided that energy. Â
 = 256/0.820 = 312.195 Newtons Â
Rounded off, this is 312 N