In general, why do smaller guns have more felt recoil? Why is recoil easier to control on a more massive gun compared to a smaller gun with the same bullet. Presumably the bullet leaves both guns with the same momentum, but the larger gun seems easier to control. Since the momentum you have to control is the same in both cases, why do we perceive less recoil on a bigger gun?
 A: Newtons 3rd Law:

When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

Momentum is product of mass and velocity. The heavier gun has more mass, so, for the same momentum, it must have less "backwards" velocity, so less felt recoil.
A: Felt recoil is partly a matter of momentum, partly a matter of force.
When a bullet with mass m leaves a gun with a velocity v, the gun must have an equal-but-opposed momentum MV, where M is the mass of the gun and V is the recoil velocity, or $$mv + MV = 0$$. If there are two possible gun sizes, $M_1$ and $M_2$, each will have a recoil velocity $V_1$ and $V_2$. If, for instance, $M_2 = 2M_1$, $$M_1 V_1 = M_2 V_2$$ and $$V_1 = 2V_2$$ Why does this matter? Consider kinetic energy. Let $K_1$ be the kinetic energy of $M_1$, and $K_2$ is that of $M_2$. Then $$\frac {K_1}{K_2} = \frac {\frac{M_1{V_1}^2}{2}}{\frac{M_2{V_2}^2}{2}} = \frac{M_1}{M_2} {(\frac{V_1}{V_2})}^2 = \frac{1}{2} 2^2 = 2$$
So the lighter gun has twice the kinetic energy of the heavier gun and this shows up in two ways. First, since both guns need to stop in about the same distance, the force applied to the lighter gun must be greater than that applied to the heavier. By Newton's First Law, this means that the lighter gun pushes harder on the hand or shoulder of the shooter. Second, the duration of acceleration must be smaller for the lighter gun, since $$S_1 = \frac{a_1{t_1}^2}{2} = \frac{{V_1}t_1}{2} = S_2 = \frac{a_2{t_2}^2}{2} = \frac{{V_2}t_2}{2} $$and $${V_1}{t_1} = {V_2}{t_2}$$ or $$ \frac {t_1}{t_2} = \frac{V_2}{V_1} = \frac{1}{2}$$ So not only is the recoil force greater for the lighter gun, it lasts a shorter time and is therefore "sharper".
A: The larger firearm has more mass, and therefore more inertia for the recoil momentum of the bullet to overcome.
Also, small firearms may be more difficult to secure a good grip on.
