-6.01mm x 355-400mm (Like cetane said)
-SHS Plastic Piston with full metal teeth
-SHS high Torque motor
-M160 (When new this may be a little higher than what you want but after a 1000 shots it will most likely come down. You can always cut it down if needed)
-Mosfet-You can get a copy of the Burst Wizzard 3 off of Aliexpress for about half the price. They seem to work well. It has a sniper delay (0.2 sec increments. So it can go as long as you want it to go) and can be wired fully into your trigger contacts as a proper mosfet should be or if you want you can use plug it in and the sniper delay will still work. (PS. I am pretty sure the BW3 has more features than the BW4)
-You can stick with the normal gears but they will be under a lot of stress and so will the motor. Everything will be working hard. I would advise using a ratio of 100:200 (I think this is a ratio of about 24:1). The problem with 100:200 gears is that they are all helical gears which I hate. They can be a pain shim.
You will most likely find that gearbox will not last too long (If you are lucky, it may). The front of the GB will most likely break off. Unlike a normal V2 or M4, you cannot use a mag well brace to strengthen it.