I cannot remove the forend cap on my 391. What is the problem?

Forend Cap A391

Most likely the forend cap is rusted in place. The forend cap on the Beretta 391 series semi-automatic shotguns needs to be taken off and lubed on a regular basis. Gas residue will accumulate in this area. This reside is both corrosive and hydroscopic. Without regular cleaning, the cap will rust in place. Also, the cap is spring loaded and it may lock in the compressed position allowing the barrel to beat back and forth causing the gas cylinder to separate from the barrel.