I just checked, a Goex "capfull" (at least one of the ones on my particular batch of cans) is about 100 grains.
You were shooting about 300 grains of a powder that is really too fine for a 3/4 inch bore gun.
I suspect the lack of a projectile is all that kept the pressure down to the point where you were not killed.
2F possibly for blanks, 1F definitly for live charges. If you use wads, they increase pressure just as though it is a "live charge". If it weighs the same as a ball, it is a ball as far as pressure goes. It may actually be more than a ball if it is pressed in really tightly.
Tropico is right about 1 " guns being fun... He and I both have Brooks-usa Dictator mortars, and Tropico has several other 1 inch guns as well. The Dictator made by Brooks sounds like a 12 gauge shotgun using way less of a charge than you are using, and the walls are about an inch thick solid brass.
The thin-walled gun you bought on e-bay is not going to do what you want it to do. If you try hard enough to make it do that, it will blow up and you will probably get your name in the paper in a less than desirable way.
Check with the barrel makers on this board, and look for something between 1 inch and golf ball size that is well made and still within your budget. You should be able to find something quite loud, quite safe, light enough to still be portable.
