Air hammers are essentially designed to be on or off, while rivet guns are feathered anywhere from a single hit to continuous hits. Some of the better hammers out there have feathering triggers, but they are not necessarily consistent in the way that they operate.
BTW, if you shoot a lot of solid rivets, a heavier gun and bar are often better ergonomically due to lower recoil to the hands and wrists. We used to shoot some solid Monel rivets with a 9X and a 25 pound bar. Bucker had to hold bar overhead, while the shooter had the gun at knee height pointing down.