M4 recoil and rate of fire

It's important to find the best balance between buffer and buffer spring your specific gun and bolt carrier weight as these affect three very important aspects of an M4 GBBR: Recoil, Gas efficiency and cycle rate.

Recommendation for typical setups (assuming compatible)
 * Buffer: rs Carbine weight. H1 and sometimes up is an option when prioritising recoil.
 * Spring: ViperTech or Waldo Customs "Snappy Boi"

Self explanatory, it's the felt recoil when you shoot the gun. The weight of the buffer (together with the weight of the bolt) is dampened by the recoil spring, more weight and less spring resistance means more powerful recoil.
 * 1) Recoil

If you're building for maximised recoil consider getting an rs buffer tube if your platform can fit one since airsoft tubes can break from the force.

Gas is used to propel the bb and then to move the bolt. A heavier buffer or stronger spring requires more gas to start moving and thereby lowers gas efficiency, while a light buffer and soft spring has the opposite effect.
 * 1) Gas efficiency

A stronger spring or a lighter buffer makes the bolt return faster to load a new bb, giving you a higher rate of fire.
 * 1) Cycle rate (Rate of fire)