I'm tryin to understand the physics..

The ammo maker had this warning and the gun has a note on the barrel - "No less than 120 gr. bullet"

It seems to contradict. But they'd not make that statement lightly. They want your money, after all. Not at the expense of folks poor mouthing their product though. I'd think a light bullet would have less recoil. A heavy one a longer recoil impulse. Maybe it's the "speed" of recoil?

The point is a bit moot. It hurts bad enough you are not going to shoot it much anyway. But you'd not want to lock up the gun with a pulled bullet if you truly needed it. .38+P's are looking like the hot ticket anyway. That way the whole family can use them. My wife carries the .38.