Very interesting, especially the part where you mention ammo with a lower muzzle velocity can function better than higher velocity ammo. I don't recall the exact wording but the comments in the reloading book I have seem to suggest the recommended powders have a slower burn rate than typical pistol loads.

After seeing your gas cylinder diameters and after measuring the O.D. of my '5 in 1 tool' I have zero confidence in the numbers I've reported so far for my barrel. I'll wait for a more reliable measurement before throwing out another diameter estimate.