Question

Why should a gun be held tightly against the shoulders ,while firing?

Answer 1

THE REASON:
According to the law of conservation of momentum ,the total momentum before firing should be equal to total momentum after firing so according to newtons third and law of conservation of momentum the gun should recoil in opposite direction to avoid this it is tightly pressed to the shoulder

hope it helps, have  a nyc day

courtesy:fire furry
any doubts?comment below

Answer 2

The reason can be found in the newtons third law of motion.

NEWTONS 3rd LAW: To every action there is an equal and opposite reaction.

and conservation of momentum.

When a force is applied on the bullet while shooting,The bullet provides an equal and opposite force on the gun.

Accoeding to Conservation of momentum:
mv=MV     
m=mass of bullet
v=velocity of ullet
M=mass of gun
V=velocit of gun

mv=MV
V=mv/M     ; With this velocity gun will be pushed back.

So if u dont hold the gun tightly, the gun will be pushed back and may hurt the shooter.