Using a powder in the Unique/Universal/20-28 burn rate or slower, 391s will cycle even sub-1200 fps 3/4 oz weenies. If your gun does not work with those powders get a thicker (red) gas valve relief spring from Coles. My 391s are cycling perfectly on just 12.8 gr 20-28 and stock valve spring.
Interstingly, in order to make the gun work with Green Dot I must approach a 14 gr charge weight. Pressure at gas port (not chamber max psi) drives gas guns.
I was able to get my 20 ga Montefeltro to cycle them at 1200 fps, or greater.
