I may be way off in my thinking, but I would think barrel length would play a role in your powder choice. Maybe JB, Aussie or one of the other writers will chime in.
Seems to me that if you had a shorter barrel a faster burning powder would be benificial. Again, im only guessing, but you want all the powder to be burned before the bullet leaves the barrel. If you had a slow burning powder that was effecently being burned in a 24 or 26 inch barrel, it might not all be burned at the 20 inch mark. Thus causing excessive muzzleflash, which I would say doesnt hurt anything, but may be leaving FPS on the table. Were if you burned a powder that was effecently burned at the 20 inch mark then you would be wringing all the FPS available. I may be wrong in my understanding of powder performance.
.............Badley.............First. A rifle barrel length of 20" or even down to 16" shouldn`t play a role in determining what powder is used or should be used. As with any rifle, the rifle itself decides what it prefers for best accuracys, whether the powder be a slower burn or be a faster burn.
It is logical to assume, that if using a slower burning powder in conjunction with a shorter barrel, there will be more unburned powder granules which exit from the muzzle. There is NO unburned powder. Instead, what we see exiting from the the shorter barrels are the "remaining leftover gases" or what is known as muzzle flash. All the powder granules, whether from a slower burn or from a faster burning powder, is consumed upon ignition before the bullet exits the barrel. Compared to the faster burns though, the slower burns will give a larger area of muzzle flash.
As far as leaving fps on the table? Well, based on extensive chrony testings, my little shorty 16.5" tubed carbine, runs on average only about 4.5% behind two 24" barreled 300 WSMs that were also tested side by side along side my carbine using the same identical loadings. By comparison, the OP`s 20" barreled `06 would give up less a percentage to a 24" tubed 30-06.
Will leaving 4.5%, 5%, or even 5.5% in overall velocity on the table, cause anyone not to be successful in the field given the same killing ranges? I think not. The game won`t know any difference.
