Action-reaction forces act

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

State whether the following statement is true or false:
In any interaction,there are always at least two forces in play.

$A$ bullet of mass $m$ is fired from a gun of mass $M$. If the velocity of the bullet is $v$,derive an expression for the recoil velocity of the gun. On which principle is this based?

Two friends on roller-skates are standing $5\, m$ apart facing each other. One of them throws a ball of $2\, kg$ towards the other,who catches it. How will this activity affect the position of the two? Explain your answer.

$m_{1}$ and $m_{2}$ masses of two blocks are connected by a light spring on a smooth horizontal surface. The two masses are pulled apart and then released. Prove that the ratio of their acceleration is inversely proportional to their masses.