1. (well its not really a problem) because of the size of fingers varries so much it would only work for you.
2. The fireing mech would have to use wires of some sort, that are taunt and when tightend compleatly fired. If they are exposed and they get a bump, they will go off.
3. It will be difficult to make one that rest on your arm, take up a little space while still retaining accuracy.
1. Well the idea of home-made guns for me is to customize it so it fits me perfectly. So, if I were to make a rifle, I'd size it so it'd fit perfectly in my own hands, I don't need to worry about others using it cause I wouldn't let anyone else touch mah babeh.
2. The firing mech would use rope. I got that idea from the Double Shot. It uses ropes and pulleys (I think it uses pulleys...). It doesn't use them for the trigger, but it's where the idea came from. Also, this gun won't be constantly cocked. It's a back up gun, for use when you're out of ammo and need some quick cover fire to escape to reload your main. A bump would only set it off after you've cocked it, and by then, I'd hope you're already aiming it at someone you want to hit. The rope triggers also won't be worn like rings. They'll be resting on the middle knuckle of your finger so you can put more leverage into firing, and reducing misfires.
3. The straps will have to be tight around the hand, and it can't restrict the movement of your fingers when it's uncocked. I'm trying to avoid using the arm, this is purely supposed to be a hand-mounted weapon, to give you the freedom to move your hand. The range and accuracy on it wouldn't be anything special due to the space restriction, but it'd both be an asset as quick cover-fire and it'd sure be one hell of a novelty. Imagine shooting with your pinky first. Then your ring finger, middle finger and you'd still have your index finger cocked. You make a gun with your hand, point at someone's head, and say, "Bang." and with the miming of pulling a trigger, you nail him in the goggles. That alone would make my day.