Just got a new 11-87 on clearance at Walmart for $249 +tax. It's got the 28" barrel and I'm trying to figure out what length mag extension tube to put on it to get as close to the end of the barrel as possible without going past the end of the barrel. I briefly searched but came up empty. Thanks.

Here's a quick helper... then inside that "length", divide the length by a standard 2-3/4 or 3" shell and take the lower figure since you have to leave room for the spring to compress, magazine follower, etc.