Choosing a powder for carbine barrel

[jpuke]

I know the topic of choosing a powder can be a lengthy one but I'm looking for some general guidelines.  I read an article a few years back that said that you should pick the powder that gives the highest velocities regardless of your barel length.  That logic doesn't really make sense to me, - if I pick a slow burning powder because the data says it gives high velocities with a 24-26" barrel, and then I shoot the same load out of my 20" Rem Model 7, will I get better velocities than I would with other powders or do I just end up spewing a lot of unburnt powder downrange?  Or does using a faster powder work better because it needs less burn time (and barrel) to achieve its highest pressurel?  Maybe I answered my own question, but  little help would be appreciated.  I know that I'm sacrificing velocity in having a carbine, that's part of the deal, I'm just wondering if I should pick a barrel length-specific powder.

[Nobade]

It really depends on the caliber you're using and its expansion ratio. A 45-70 doesn't really care how long the barrel is, it gives about the same in 20" or 24". A .264 Win or other big case/small bore round really likes to have a lot of barrel and slow powder. But in a short barrel you can sometimes (but not always) equal the velocity you'd get with slow powder using something faster to reduce muzzle pressure and noise. But I wouldn't go too far out of my way to optimize it  unless you have a good powder selection in stock and feel like experimenting.

[PaulS]

Jpuke,
The cartridge capacity in conjunction with the bullet weight determines which powder(s) burn most efficiently in any weapon. Barrel length has very little to do with the equation. A longer barrel will give you more velocity and a short barrel will give you more muzzel flash but the powders that give the highest velocities in a long barrel will also give the highest velocities in short barrels.
The reason for this is that the powder is actually burnt in the first few inches of barrel (most is burnt before the bullet leaves the case) and the heat / pressure / time curve regulates the accelleration of the bullet. A faster powder gives maximum pressure very early and drops off fast while the slower powders produce a higher pressure longer with the same peak pressure allowing the bullet to accellerate for a longer time. (even in a short barrel) The NRA and others have tested this over and over with a single barrel, cutting an inch off at a time and testing with different powders for maximum velocities. The optimum powders didn't change with barrel length.
PaulS

[jpuke]

Thanks for the responses.  I've been shooting a 308 Winchester with Win 748 for a while and recently I've been working on a load for a 170 gr. FP (Hornady) with IMR 4350.  I actually shot that bullet today behind 46.8 grs of the 4350 and got .75" groups - I think I found a winner, and with what you guys said I'm not going to worry that I'm using a slower powder.