Fast vs Slow Powders

[PecosRiverM]

Why would you want to use a faster powder in a rifle round?

Powders I'm looking at:
IMR 4831
H 4831
H 4350

Rifles I'll be using:
.243 95gr Nosler BT
25-06 115gr Nosler BT
.270 130gr Nosler BT

My Dad is already using IMR 4831 for the 25-06. But would one of the others be a better choice?

I've still so much to learn

 

[amlevin]

If your rifle has a short barrel you will want a faster powder. That is, unless you want a large amount of unburned powder leaving the barrel with the bullet.

I have a CAR-15 that has a barrel only 11.5" long (plus the flash supressor to make it a legal 16" barrel) and it performs best with quicker powders like AA2230. H-335 and 748 leave a lot of residue and the rounds 'clock' a lot slower.

 

[BsChoy]

According to Hodgdons site anyone of those is a very good powder for any of those 3 rounds. It seems 4831 in either flavor would be the way to go to me but H4350 doesn't lag too far behind at all.

 

[Dave P]

You want to use a powder that burns 100% just as the bullet reaches the muzzle. This allows the gasses to accelerate the bullet to its max speed, applying force for the full length of the barrel.

If it burns too fast, you will not get maximum velocity, since all expansion has occurred before the bullet reaches the end (no more force for acceleration).

If it burns too slowly, like the man says, the unburnt powder will be expelled as a fireball, and the bullet velocity will be lower than it could be.

Now, the peak pressure generated is another issue affecting bullet velocity. Obviously, you want to stay within the SAAMI limits.

I'll bet someone else can give a more detailed explanation...