[rotorwrench]

1 and 1/2 the diameter or slightly less is acceptable for nearly all driven rivet applications as to the protrusion of the rivet prior to setting them. Too long and they will work harden before setting to near 1/2 D after the buck. Small rivets will nail over if too long in protrusion. This would apply to all rivets whether bucking them to a flat or round profile buck tail. Bucking bars with a round profile indent will only buck till the bar hits the substrate metal if it will buck that far.

The old Cleveland Pneumatic or Cleco guns were the best but difficult to find in this day and age. I think there are modern ones with the trigger adjustment on the bottom of the hand grip but they might be difficult to find. Most use an adjustable air valve that connects to the hand grip instead of throttling them with a regulator at the compressor or air manifold. My compressor runs at 125 PSI but the hose drops that to just less than 120 PSI. Dropping the pressure at the hand grip eliminated drop lag that happens when you first pull the trigger. this is caused by the length of he air hose.

Like any tool, if you use it often enough, you develop ways to make it work better for you. as you gain more experience with it.