Other models of the 870 Tactical come with different options for the stock (basic synthetic, pistol grip synthetic, speed feed synthetic and Magpul), different barrels with ghost ring or rifle sights and some have the receivers drilled for rails and optics. The barrel of the Tactical in the pic you provided is drilled for a choke as is evidenced by the breech adapter and has a rifle type forward sight. The magazine (unseen in the pic) is the standard 4 round with a two round extesnsion (seen in the pic) with a sling clamp. This tells me the barrel is interchangable with almost every other 870 barrel on the market.I have an 870 Express Tactical. It does, in fact, have a cylinder bore "breaching choke". It screws in like any other choke, and the "breaching" part extends out beyond the end of the barrel.
I don't think he cut an inch off, I think he just unscrewed the breaching choke and either put in another standard choke or left the threads exposed inside the barrel. The exposed threads would get damaged the first time he fired it, but it's not like he was trying to take care of this thing.
Alexis only sawed off about an inch from his barrel making it even with the magazine cap and taking off the front bead sight. I can understand wanting it short to be easier to manipulate in close quarters, but I imagine he did it to prevent the bead hanging up on a piece of clothing in the event he had to whip it out in a hurry.
There is no telling what was going through his mind. He was definately troubled to feel the need to kill, but then he goes and destroys a perfectly good shotgun. How could he do such a thing?