Bull: I don't know the barrel length of that gun, but you can use a formula to determine the maximum amount of powder that can burn in any barrel.
Its: Bore diameter x Pi R Squared( .50( Diameter) divided by 2( to get the Radius(( R ))= .25, times itself( .25 x .25= .0625 times Pi( 3.1416)= 0.19635. Multiply that figure times 11.5 grains (per cubic inch), = 2.258, then multiply that number times the length of the barrel to get the maximum load of powder that can be fired in your gun.
To effectively burn 90 grains of powder you will need a barrel that is 39.8 inches long. I suspect that the barrel on your gun is shorter than that. A 36" barrel will burn 80 grains of powder. A 34" barrel will burn 75 grains of powder. A 31 inch barrel will burn 70 grains, and so on.
You can improve compreesion, raize chamber pressure, and create more barrel time to burn powder by using wads and fillers to increase the mass in front of the powder, or shoot conicals, that weigh more( more mass) than does a PRB. The conicals will eat up more powder but travel slower. Using the wads or fillers over the powder will allow the barrel to burn more powder while keeping the weight of the PRB the same. I prefer using fillers, as they are lighter in weight, while still requiring both compression and burning to push the PRB in front of them down the barrel. Because of the lighter weight, you don't get any appreciable increase in recoil. Shooting conicals definitely increases recoil! I can't tell you how much more efficient a barrel will be at burning more powder using the fillers. I have not done experiments on this. However, you can put newspaper, or an old sheet on the ground in front of your shooting bench to catch unburned powder, and try various amounts of powder with the fillers to see how much more burns and how much doesn't. My personal experience tells me that the same gun will burn more FFFg powder for its barrel length, than it will using FFg or Fg. I tested that in both a rifle, and then shooting blanks in a cannon! ( Sometimes doing these kinds of tests are just downright fun!)
Paul
