Tactical Trijicon ACOG TA01 Style 4x32 Cross Sight Rifle Scope + Kill Flash
TROBB Solid Aluminium Dovetail Power Block with Gmac hollow probe and G-mac Bolt
Gmac Brass Power Adjuster Spring And Guide/Drilled Out Standard Valve For more air space
Hammer Mods//Gmac 30% Lighter valve Spring/Gmac Brass Safety Catch//Transfer port drilled out
Full Gmac Trigger kit/Gmac barrel cut to 9inch/Weihrauch Silencer/Custom Home Made Grips/
New TRACER LEDRAY GL2 LAMP 90m Range Beam//HI-Pac
...always increases as the reservoir volume available to the valve increases. That's because, with a small reservoir volume, pressure begins dropping rapidly the instant the valve opens. But with a big volume with a good flow path to the vlave, the stored gas backs up the gas expended with the charge, for a higher average pressure and therefore more velocity.
Maybe that fact plays a role in producing the power difference you see between 12gr and bulk?