Yes, the PC feature is located on the barrel and works like a pressure relief valve to prevent over-gassing. The 26" pressure-compensated barrel has larger gas ports and the valve will open with hotter loads and vent some of the gas. The 21" slug barrels are
not pressure compensated and have small ports to work with full power loads. You are correct, that barrel won't allow functioning with reduced loads.
The shortened barrel measures 18-3/4 inches, cut to line up with a rib support and rethreaded to take the RemChokes. It cycles the Federal LE reduced-recoil slugs and
9-pellet 00 buck loads perfectly, as well as full power loads including
Brenneke short field magnum slugs. Due to the reduced barrel length past the ports, it will no longer cycle light target loads or the lighter
8-pellet 00 buck loads, but that is of no consequence for its intended role. With the Federal LE reduced-recoil loads, it is both a pleasure and a lot of fun to shoot.
The stock was also shortened by 1-inch and now has a 13-inch LOP - this is as far as the stock can be shortened due to the recoil spring. The OAL of the entire shotgun as shown is 35 inches.