Here's what I've found with Shockey's Gold (loose FFg) in my Mustang. It is very accurate, but very low pressure/low velocity. For example,105 grains volume of JSG under a 300 grain Gold Dot in a Harvester short blacksabot gave me a five shot average of 1264 fps. 120 grains gave me 1381 fps.

120 grains of GOEX FFFg gave me1641 fps.

100 grains of T7gaveme close to 1800 fps.

The lowest charge of T7 I've shot with this bullet was 75 grains, and it gave me 1635 fps. - more than250 fps faster than the 120 grain JSG load.I suspect I would have to back T7 down to 60 grains to get as low a velocity as that 120 grain load of JSG.

I haven't shot Pyrodex with the same bullet as the above loads, but based on its performance with other 300 grain bullets I'd guess 100 grains of Pyrodex RS would be somewhere around 1500 fps.