This is an image of the RobArm trigger next to an AR/M16 trigger (AR on Right)
The an AR hammer, FA, next to a RobArm hammer (AR on right)
and again from a different angle, again AR on right.
Hammer height looks about the same, through calipers, when looking at max length. The biggest difference appears to be the face angle of the RobArm hammer. It swings further forward to strike the firing pin and I'm not sure if that would prevent an AR hammer from being used, though it might need to be reprofiled and re-hardened. I might grab a spare semi auto hammer and have a play next month.
The stock Robarm trigger is really crap though, which is a shame considering the rest of the rifle. On the hammer you can see the casting marks and i've got a trigger with casting marks on the action surface. I can't understand why an AR trigger wasn't used unless it's some idea of cost saving. What I don't understand is that a good trigger can improve a bad rifle, and bad trigger can kill a good rifle, so why screw this up so bad and take so long to do something about it. A good match AR trigegr is probably one of the best triggers this side of Olympic mathc pistols or electronic trigger so why not use one.