Question

two identical bullets are fired one by one by a light rifle and another by heavy rifle with the same force.which rifle will hurt the shoulder more and why

Answer 1

The heavy rifle will hurt more because heavy rifle will produce more force than light rifle.
we can use Newton's third law of motion in it- To every action there is an equal and opposite reaction.

Answer 2

Two identical bullets are fired one by one by a light rifle and another by heavy rifle with the same force.which rifle will hurt the shoulder more and why

Same, because mass of bullets and force of bullate are same