I make my SAW/LMGs shoot a little slower than normal ROF...just like that effect.
A buddy who rarely ever posts here has some screaming setups...mosfets, brick sized LiPo's, dual sector gear, high speed/high torque (i.e. M170-190 range springs). Freaking crazy. Biggest issue he has is having the mags keep up with the guns.
He just likes building things and pushing limits. Which is fun...and really expensive...to do.
So far as I know...he never shoots anyone with those setups. Not really a sportsman-like thing to do.
His 400fps propane 1911 (or 2011...can't remember) is a really neat build too.