Super simple, school-level question I just can't wrap my head around.

I was trying to explain recoil (in relation to firearms) to my younger brother, and found myself stuck in the physics behind it.

Let's say we have two guns, firing the exact same projectile, with the exact same mass, at the exact same velocity. The only difference is, one gun is 1 kg, the other 2 kg.

How do I calculate the difference (momentum?) in how each gun is effected by the force of the projectile being fired?

Again, I realize this is school-level physics, but I'm afraid I haven't used this in years, so I'm a bit rusty.