generally, air hammers (very close to rivet guns, which are very dear to my heart) come in different 'powers.' In the rivet gun world, you can get anywhere from a 1X to about a 9X rivet gun. The bigger the 'X,' the harder it hits. The biggest difference I have noticed physically is length of the 'barrel,' i.e. how far the hammer travels. At about 5X, you switch from 0.401" tooling to 0.498" tooling (diameter of the piece which actually fits inside the barrel of the air hammer). You can buy adapters to use either tooling on either size. They make some true monsters, if you need a bigger hit--tools used for forming heads on iron rivets on structural steel. In general, you can find an rivet gun with as much, or as little power as you need, for just about any job.
So, it is generally like this:
Smaller X = shorter barrel = lighter, faster hits (more hits per minute).
Larger X = longer barrel = harder, slower hits (fewer hits per minute).
Kev