The Model 1100 and Model 11-87 barrels are NOT interchangeable. Here are some differences, and why they are not interchangeable:
1. The gas cylinders are welded in slightly different locations. This is partly because the Model 11-87 has a longer magazine tube. The Model 11-87 also has 2 gas orifice holes. Only the 2-3/4 inch chambered version of the Model 1100 will have two. The Model 1100 Magnum barrel has only one.
The Model 11-87 is pressure compensated. This is what allows it to cycle both 2-3/4 and 3 inch shells. To accomplish this, two additional holes are drilled in the top/front of the gas cylinder, and covered with a tempered steel spring. There is a relief cut made into the gas cylinder to accommodate the spring. The Model 1100 gas cylinder will not have the relief cut or the spring or the two additional holes.
2. The extractor relief cut on the Model 11-87 uses a wider extractor. Therefore, the extractor cut in the breech end of the barrel will be wider than the Model 1100.
3. The barrel extension on the Model 11-87 has a slightly longer barrel extension. This is the part that connects directly into the receiver.
There are actually a couple other slight differences. If you were to use a Model 11-87 barrel on an Model 1100, basically the barrel will slam back into the receiver every shot. Also, the bolt velocities are increased. Eventually, the front of the receiver will crack, maybe even chip off.