It depends on the type of powder, the age and caliber of the cannon, the condition of the cannon, the projectile, etc.
First, find a copy of the owner's manual for YOUR pistol. Second, use ONLY black powder, or a modern black powder substitute, such as Pyrodex. The ROUGH rule of thumb is one half the bore diameter in grains of powder. A .32 cal would use ABOUT 12-16 grains of powder.
Typically 50 to 75 grains of ffG powder.
It is illegal to purchase or possess black powder for use in modern firearms like the Barrett M82. Black powder is considered an explosive and is highly regulated under federal law. It is recommended to use commercially available smokeless powders specifically designed for these firearms.
In my short barrels I use 15 grains of 3F black powder. You can fiddle a bit with the amount +/- to do what you need, but I found it to be a good load for up to a 4" barrel.
Either FFFg black powder or Pyrodex P may be used.
40 grains of fffG
In California, individuals without a permit can legally possess up to 5 pounds of black powder or smokeless powder for personal use. However, it's important to check with local laws and regulations regarding storage and transportation of gun powder.
Depends on the cannon- the bigger the diameter of the barrel, the more powder it would take. A cannon of the type used as mobile artillery during the American Civil War would use an average of 2-5 lbs of powder per shot, depending on the size, and type of projectile.
Black powder is not illegal to possess or use in all circumstances, but there are regulations and restrictions on its use, especially in certain areas or for certain purposes. It is important to check local laws and regulations before possessing or using black powder.
Being an in line you should have no trouble with pellets.
Larger grain black powder, such a fG or ffG. Black powder substitutes, such as Pyrodex are not intended for use in propelling fireworks. Pyrodex R could be tried- rmember it is measured not by weight, but by volume.
Most recommend is 3F powder, either true black powder or the "synthetic" or black powder substitute equivalent and between 25 and 30 grains should due nicely.