edelsite

One of the big reasons for sub-powders is they are not suppose to be as corrosive as real BP. Which might be true but you still have to take care of your metal.

Another reason is that black powder is governed under the rules that is an explosive, while the subs are in a less strict category...

Most subs require a greater heat source to start the powder buring and most develope greater pressure but achieve simular velocities...

Most subs are developed to be direct replacement volume for volume of true BP. But even true BP does not follow that rule as different BP's have greater and lesser ability to create pressure. Triple Seven is suppose to be 15% greater in power than regular BP, and in reality it probably is around 10% greater than most BP loads... Swiss BP, a real BP, is generally thought to be the most powerful of the BP. Swiss and T7 are suppose to be about equal.

Somebody with a greater understanding than I will explain it better...