Don: for what its worth, an Old Hodgdon Reloading data manual gives these loads for a 20 gauge ML shotgun:
1. 2 1/2 dram,( 69 grains) FFg for 1020 MV with 1 oz of shot. This is described as a " heavy " load.
2. 2 1/4 dram,(61 grains) FFg for 987 fps. MV with 1 oz of shot. This is described as a " medium" load.
3. 2 drams,(55 grains) FFg for 992 fps.MV, with 1 oz of shot. This is described as a "light" load.
They recommend FFg powder for all gauges. And, for the best patterns, they recommend using the "light " loads shown.
A 325 grain .600" caliber RB is 3/4 oz. So, using any of these loads will most likely produce more velocity than that shown for the 1 oz load of shot.
I tried 2 3/4 drams(75 grains) of powder in my 20 ga. fowler, and got 1050 fps. MV.I don't want to go much higher than that, altho I intend to test an 80 grains load, which will be just shy of 3 drams. Its been a recommended load for PRB from several members here, and I want to give it a fair testing out at 75 and 100 yds. The 75 grain load shoots fairly well at 50 yards.