I bought a real nice condition shotgun recently in 20 Ga. 3" magnum with the steel receiver and I too had the problems with it not cycling with any 2 3/4" or 3" ammo. I soon discovered the black goo on the piston that seems so common an issue and spent quite sometime cleaning up the mag tube inside, reaming it with a brake hone, cleaning the piston, etc. I put it all back together and it still would not cycle but I noticed that when I inserted the piston, it went in quite tight as it hit the slotted areas of the tube even though it was cleaned up. My first thought was that the mag tube was bent or gouged inside preventing the piston from going in easily. By chance I came across a used 20 ga. piston on Ebay which I bought just to have a backup to the original. Guess what, it fit down the mag tube easily and now the gun will fire 3" ammo and cycle perfectly. It will not however, cycle and 2 3/4" ammo, low or high base.
Is it possible that somehow my cleaning of the original piston caused it to swell? Or maybe it was a little over size to begin with???
I may look into the modification of adding the 2nd gas vent hole in the barrel and having the Allen screw fitted to allow the use of all ammo sizes.
I would prefer to buy another barrel for just 2 3/4" ammo but am not having a luck finding one.