Question

why is the recoil of a heavy gun , on firing , not so strong as that of a light

Answer 1

The reason why the recoil of a heavy gun is not as strong as that of the light gun when fired is, because of a simple theory that states that the recoil speed of the gun is inversely proportional to its mass. 

 Since recoil of a gun is the reaction to the firing of a bullet from a gun, when a bullet is fired from a gun which is heavy and has more mass, the speed at which a bullet is fired automatically results in less speed as compared to a bullet fired from a light gun.
or
Because the mass of heavy gun is more because of which the recoil acceleration will be less. On the other hand, the mass of light gun is less, therefore it will have more recoil acceleration. Therefore the recoil of heavy gun will be less as compared to light gun

Answer 2

HEYA MATE !!
HERE'S YOUR ANSWER...
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Recoil in a gun is caused due to the action and reaction force ( Newton's third law )
It happens Because the mass of heavy gun is more because of which the recoil acceleration will be less. On the other hand, the mass of light gun is not so heavy that's why it's Recoil acceleration is more..

*hope it helps you!!

*being Indian*