Have ya looked at the MP5K's? V3 Gearbox in a smg sized gun!
Here's one of my guns I used to have.
TM MP5K. Hurricane FTK in that one, was pumpin out a consistent 398fps.
2nd one hehe, this was a crazy little MP5k.
During first upgrade, it was firing at 420-430 fps with .20g's.
Swapped spring so gettin a 390-395 area fps. Was an excellent gun, never once jammed, 185-215ft shots from hiding in my sneaky spots, got good kills an amazing accuracy. For the longer shots, I use AE .25g Black bb's.
They blend in with the shadows of the forest so god damn well you really have no idea where the shot came from, I love em!
Best part is, you can weild it one handed no problem, it's like a sniper pistol. With full auto capability. I used 100rd mic cap's with it, it loves them, an decent weight ratio even with battery.