what makes a powder specific to a pistol vs a rifle?
Excellent question and this "stupid error" was the vehicle by which a member can clarify some misunderstanding. See? This thread did some good already!
The powder burn rate is the biggest factor in powder selection (but not the ONLY one). I'll edit this post and link a burn rate chart after the post is done.
As the primer lights the powder, it burns, not explodes. It's a fast burn but it's still a controlled, predictable burn.
As the bullet proceeds up the barrel, it relieves/reduces pressure as the volume of the gas behind it increases. With the bullet moving, the cavity containing the gas is getting larger so pressure is dropping. Meanwhile, the powder is still burning, which increases the same pressure in that cavity. Ideally, the powder burn rate will exactly equal the loss of pressure and length of the barrel determines how long that burn will be.
Using a very slow powder in a long barrel will result in unburned powder when the bullet exits the muzzle and a fireball results. Using a very fast powder in a long barrel will result in extreme pressure spikes as the balance mentioned previously is upset.
When you load for a 2" barrel, it requires a very fast powder like Bullseye, Red Dot or WW231 so it is consumed before the bullet exits the barrel. Otherwise, we get that fireball. In a 4" or 6" pistol barrel, a slightly slower powder like Unique will be better. The classic load for a 44 magnum is 2400, which is pretty slow powder. Elmer Keith developed that 2400 load.
The best plan is to follow your loading manual data. Some of the best sources are from the powder companies. They spend millions of Dollars to research this data.
The worst case in reloading is to take a super fast pistol powder like Bullseye or 231, fill a rifle case with it and the result will be our KB or KABOOM! I doubt if any rifle could withstand that. The reverse, like old Vet did is less dangerous. He substituted a slower rifle powder in a short barrel. No doubt there will be a fireball of unburned powder but this is less dangerous. Not good....just not as bad.
I'll snag that powder burn chart.............
Powder Burn Rates
There are other factors in powder selection, too. Bullseye requires a tiny amount of powder for a 9mm or .45acp. It's easy to double charge with Bullseye because there's plenty of room in the cartridge case. A loader may choose 231 or Red Dot because the volume of powder for each round is greater and a double charge will overfill the case and make it noticeable.
A slower powder will accelerate the projectile less quickly and reduce felt recoil to the shooter.
In rifle, I use IMR (Improved Military Rifle) 4895 for most cartridges. That is a powder made of tiny tubes of powder. Recently, I changed to 748 in .223 because the IMR 4895 clogged ("Bridged") in the tiny neck of the .223. 748 is a ball shaped powder which flows through the case neck better when loading it.
Hope that helps! Excellent question!
Flash
Thank you I'm trying to soak up everything I can before getting into it.