Could you put an external spring behind the blade, within the trigger guard ? Would need to have a little guide. Or a tension spring higher up the blade, pulling it forwards ? Trouble is IIR the mk2 rapid has no trigger guard, so attaching it to the timber might get tricky.

Or pick any mag fed gun you like where rigging up one of these arrangement would be easy, i.e. with a metal trigger guard. A rifle where spare blades and triggerguards are cheaply / easily purchased makes thing even easier, as you can permenantly modify them both the mount either end of the spring, without devaluing the rifle.

Another alternative is buying a crosman 1077.. long, heavy crap trigger, multishot... but low power and not very accurate.