Guarder SP150 Infinite Torque-Up Kit for M4 / M16 Series Airsoft AEG
(Long Barrel)

"Definitely, a good buy if you don't want to buy the individual parts. Is a good quality parts kit. Very happy with the SP150 spring, which brought my FPS to 490-500. But, before you buy, make sure you have a reinforced gearbox and a motor strong enough to pull the SP150 spring. Motors pulling M150+ should be sufficient. I bought a G&P motor for this tune-up kit. If you do not have a reinforced gearbox and a strong enough motor, you have a higher chance of breaking your gearbox and burning your motor out. Otherwise, this tune-up kit is very economical. Good buy.

"Well first things first... if you are going to buy this kit make sure you have a reinforced gearbox. The parts are fairly high quality and don't use any of the "cheap" plastics or metals. The m150 spring is kind of hard to put in and keep all of the gears in at the same time, but is doable. The gears are a crazy high torque ratio so unless you have a high voltage battery, it might seem a little slow. This kit uses helical style gears so if you are going to buy an aluminum piston make sure it is a HALF TOOTH. The piston that it comes with is high quality poly carbonate but just going to be strong enough to stand up to the m150 spring.

Pros.
High quality parts
got me up to about 500+ fps :D
Helical style gears
just plain worth it instead of buying random parts individually

Cons.
Kind of hard to install but little to no modification needed
loud
not for close quarters :(
Really slow gears (but that is expected with a high torque set-up)

Overall great buy if you are going for a DMR style build. I would definitely recommend this only if you have a reinforced gearbox shell and a high torque motor. Just remember to try not to shoot full auto for it will increase wear and tear. Besides you shouldn't be shooting full auto in general with an M16 SNIPER :D