You'll find that real BP shooting is just as complex and involved as modern smokeless... You have to consider "burn rate" (rate of conversion to gas, BP is a low grade explosive in all granulations (technically the primary active component in Pyrodex (Potassium Perchlorate, combined with BP) and 777 (Dinitrotoluene) are too (although those two really are semi-smokeless powders), meaning it turns to gas incredibly quickly, compared to most solids and liquids, yet not as fast or completely as high explosives (ex: Trinitrotoluene (TNT)) and how the diameter of the charge vs the length of charge vs granulation of the powder affects that. This means that gun powders like BP only generate intense pressure for a brief moment, and then decrease in pressure until the bullet has completely exits the bore. We know from scientific study that the rate of combustion reactions exponentially increases, as the pressure increases, so long as there is adequate oxygen to oxidize the substance. If powder on a plate goes "poof" in the open air... imagine what happens when you confine it and it reaches pressures 1360 (20,000 psi)-2721 (40,000psi) times the pressure of our atmosphere (ATM)! Fortunately, BP is not nearly as erratic under pressure as modern semi-smokeless and smokeless powders are.
You need to consider your projectile, and it's inertia (which determine the pressures you are creating, BP is capable of incredibly high pressures, given the proper conditions). You have to think about your barrel length, and giving the powder distance (time) to convert to gas, and still give the projectile time to make use of the pressure in the bore to accelerate. True BP is 100% water soluble, and is very easy to clean, which is good, since only about 55-65% of it's mass converts to gas, so there is a lot of soot left (and smoke, don't forget the smoke
). The goal should always be to balance your gun, projectile, and charge in such a way so as to achieve as close to that 65% as possible (sometimes, however, to get just a bit more performance (flatter trajectory, more energy, ect.), we have to accept a reduction in efficiency). Different companies have slightly different formulations of BP, just like there are different formulations of smokeless today, so you have to decide on what works best for you/your wallet/your gun and projectile. More efficient powders will produce more velocity and pressure for their weight, and will result in less fouling.
All in all, considering the charge dimensions, and the inertia of a .490-.495 Lead Ball, 3F is a great all around choice, which is why it is the most commonly recommended granulation today (technically, 4F can be used in .50, but only in fairly light charges (I'm going to guesstimate, 30gr in modern guns not specifically proofed for 4F) as the fineness of the powder allows it to convert to gas more efficiently than even 3F, and the charge will be even shorter in length, which will also increase the rate of conversion, and therefore, pressure). about 40 years ago, many people recommended 2F, but in looking through older articles and such, I can't seem to find an exact reason anyone points to as to why that is.
The general rule of thumb for firearms is for larger bores (wider, shorter charges) (ex: .32 vs .50 vs. .72) and heavier projectiles (more inertia), you normally will want a slower converting powder, in order to keep the pressures manageable (ex: a 100gr charge with 172gr .490 ball vs the same weight of charge with a .490 694gr wadcutter (generally want a slower powder with that heavy of a bullet)). With smaller bores and lighter projectiles, you can make better use of faster powders (like 4F in a .32 with a ball or light bullet vs 3F in a .50 with a ball or light bullet vs 2f in a .50 with a heavy bullet).
As always, make sure your gun is proofed for what you want to do with it, even if that means you proof the barrel yourself (DO NOT PROOF IN THE STOCK!!! a 2x4, caliper, notepad, baling wire (wrapped in tape to protect the finish), cannon fuse, and a backstop makes a cheap and effective proofing setup (just get behind something). With hazmat being required for all gun powders today, I just stick with real BP, it's easy to clean, easy to work with, gives me all the performance I need... and how else can I get the wife to complain about the smell when I clean guns after shooting lol
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