|The gauge of a shotgun is the size of 20 or 12 lead balls that make a pound to fit in the barrel.|
The lower the number, the more powerful the gun.
The 20 gauge should have less recoil than the 12 gauge but it really comes down to the weight and ammo used.
The common gauges in the US are:
The 12, 20 and 410 are the most popular.
The 410 is the diameter of the rifle bore or .410 inches.