Looking at my nosler number four manual under the handgun loads it shows the same powders for the 14" barrel as for the 26" barrel, just slower velocity. It looks like n150 and rl15 are the winners for speed at 2684 fps. H4895 and imr4064 were the next two for speed at just over 2600fps. I would guess 4 more inches would just make any of these better. Big game and 748 are 2 I would look at as well
