The Flat Nub by Kingdom of Airsoft. 
Our own precision cut Flat Nub has been installed in over 6000 Airsoft guns over the last decade since its initial development. 
Starting off as the ideal nub for use with our R-Hop installations, these have been expanded to work with all buckings and most hop chambers. 
The Flat Nub applies a more even pressure along a greater surface area allowing even standard buckings to provide more friction and efficiently float heavier ammo. 
Thanks to the nature of the Sorbothane material, these nubs can more quickly reset than traditional options allowing improved consistency with faster rates of fire. 
The Flat Nub can be set up with the stock hop arm but for best results, the concave nub holder found on most hop arms should be machined or sanded flat. 
These work superbly with PDI W-Hold and Prometheus Purple buckings without any modification required to the bucking but are also compatible with virtually all buckings and hop chambers. 
Thinner versions can be made upon request for the use with AK sliding-arm and AEP/CMG chambers. 
We recommend these nubs paired paired with a standard buckings and most for use with up to 0.30g ammo and heavier when paired with R-Hops or R-Hop style buckings. 
The Flat Nub measures approximately 6mm x 4mm x 4mm bus customer sizes are available upon request.
We recommend applying a thin layer of silicone oil to the nub to allow it to move smoothly in the chamber.