As a general rule of thumb barrels longer than 18" yield higher velocity by 25-50 ft/sec per inch. However this is real general. The only test I've ever made was on a 26" barrel shortened one inch at a time to 19" and the velocity was faster at 19" than at 26"...this was a 12 Ga slug BTW. I've also read accounts of this happening with a 6MM Remington.....

For scope sighted rifles there is no data to show accuracy differences in barrels from 16" to 26" for typical hunting and varmint rifles.