I don't think that you will notice much difference past about 24 inches. Muzzle loader powder does not burn like center-fire powder. It is more of an explosion. Long barrels used to really help with open sights, as it improved the sight radius, but now with the scopes, the short barrels are all you need.
I have used 777 for years in my guns with good results. This last year, I switched to Allliant Black and I REALLY like it.
