If the barrel is chambered for 3-1/2", all the gas ports in the 11-87s are the same size, and have the gas relief system. Note some 11-87 target barrels are the same as 1100 barrels and don't have the gas relief system, and I think some of the shorter deer barrels are the same way. There is a big difference in gas ports between the gas relief system barrels and the non- gas relief barrels; like 0.050". If you run into cycling problems with a load and a gas relief system barrel, just stick in the barrel seal activator that the SuperMag is supposed to come with for shooting lighter loads. Just don't forget to take it out if you go to heavier loads. If you don't have the barrel seal activator, a fatter O ring will do the same thing. It just blocks the flow of gasses to the relief ports so it is all available to work the action.
I only see a regular a 21" Model 11-87 barrel listed by Remington, not a non-rifled SuperMag 3-1/2" chamber version available separately, and they say the SuperMag barrels will not interchange, unlike the 870s. If you want to go to 23" or 21" with your barrel, going to depend on where the rib post falls, I wouldn't be too concerned, but be advised you will probably not be able to get RemChokes re-installed because the outside diameter will be too small. But Colonials or TruLocks and such will fit. They work fine. The difference between the $110 Super Deluxe Extreme Long Range Monster Turker Slaughterer choke tube and the $15 RedHead Extra Full off the peg at BassPro isn't nearly as great as they would have you believe.
