5whiskey claims most factories test fire out of 26" barrels.
I disagree. Check out the following starting on page 135 and note the length of SAAMI test barrels used at factory test facilities to qualify their ammo to SAAMI specs:
The vast majority of 'em are 24 inches long.
5whiskey also claims most factory ammo is loaded too far under max.
Go to page 13 in the above SAAMI specs and check out the cartridge-bullet/weight-velocity-pressure numbers then compare them to factory published data on their ammo.