In addition to a long fun day at the range I got to test some new (to me) bullets and powder in the Hi Point. Switching from Barrys plated to some RNFP jacketed bullets in 155gr weight was an eye-opener. Now in all fairness, Barrys does not recommend using their plated bullets at muzzle velocities greater than about 1200 fps as the plating is prone to come off. This means that they work fine for nearly all pistol applications in eliminating lead fouling.
When the plating starts coming off, however, accuracy goes the way of the wild goose, and muzzle velocity becomes much more widely variable. In the Hi Point, the longer barrel boosts muzzle velocity 250-300 fps over what you would expect from a pistol of the same caliber putting the speed well above Barry's recommendation.
FWIW: Using Titegroup, my favorite one-size-fits-all powder, the 155 gr jacketed bullets delivered an average of 1418 +/- 23 fps and group size of 3MOA shooting off sandbags.This is about 60 fps faster than the plated bullets, and 3 MOA closer groups. This accuracy references loading Barrys bullets at sub-minimum loads resulting in MV's around 1250-1300 fps. If you load upward, the "groups" quickly expand to the point where they were measured in feet at 100 yards. It's not the gun, it's the bullets. Use jacketed above 1200 fps.
Switching to Power Pistol confirms the thought that slower powder would work better in a longer barrel with MV up to 1579+/-27 fps and about the same accuracy.