One thing I've wondered about: would it be practical or advisable to design a rifle with a trigger that worked like an electric typewriter's "X" key [push lightly for one strike, mash for automatic?] I would think that in a surprise situation, it would be difficult to flip a selector lever in the heat of the moment, while applying an extra 5 pounds or so on the trigger would be easy if not automatic.
Been done. At least two weapons are like that. The Czech VZ-24/5/6/8 submachine guns of the late forties, and the Steyr AUG -- which is the best of the bullpups, IMHO, but has a lot of detractors. Most of them have only shot the semiauto one and I have only shot the military one so I dunno what their problem is. I was concerned about the plastic magazine and mentioned it to the Steyr-Daimler-Puch rep. He whacked it with an M-2 .50 barrel, and then had a Sheridan tank do a pivot turn on it. It was pretty scuffed up but worked fine.
I have thought about using an electric trigger to avoid all the problems that come from mechanical ones (mostly disturbing the steady hold of the weapon). There was a gun in American Rifleman once that was set up that way -- it was designed for a handicapped man who did not have the strength to handle a normal trigger but still loved hunting. IIRC it was a bullpup on a US Enfield action with a laminated wood stock -- very exotic thing! This would have been about 1972/3.
d.o.l.
Criminal Number 18F
There've been several such designs, many on Beretta submachineguns, on the reasonable assumption that in the most usual circumstances, a carefully aimed shot is more likely to connect, but in the event of gut-wrenching panic, the operator only needs to yank the trigger as hard as possible and hang on.
The Ingram M6 SMG was one of the few American weapons to offer the feature, but trigger mechanism components that offer that feature have been developed as a replacement for M16A1 mechanical components, and can be fitted in the M16A2 or M4 carbine as well.