I asked the same question a year or so ago to some industry folks and got a variety of answers with several variables such as pellets vs powder, sabot vs full bore bullets, maxium velocity vs accuracy. Twist rate of barrel.
In a nut shell most of them thought 22-26 was a good length, check out your blank size for your barrel if > than 24inches is required you will pay more. I use a 24inch barrel for sabot in a 45 and 50 cal muzzle loader and get good expansion and trajectory for 200 yard shots.
In the Encore Full bull barrels over 22-24 inches will not balence as well as the shorter barrels, My 24 inch is a good size for velocity, accuracy, balence etc, if you want a longer barrel see if you can get a more tapered size blank. But I would still not exceed 28 inches as the max.
DC 195 gr
120 777
25 acp
24inch Bulberry barrel
trigger job 1 .5 lbs
4x14 scope
Of all my loads this is the most accurate listed above about 2 inches at 200 with an occasional flyer.
out of the group at 4 inches. I have shot about 4 deer over 140 yard but less than 210 yards with this load and the longest recovery was 70 yard but the other 3 were less than 40 yards. so velocity must still be good.
Only my opinion
TC