From the Remington website:
What is the difference between the Model 1100 and Model 1187?
In 1987, the new gas compensated 1187 was introduced. The 1100 and the 1187 are similar in design. Barrels are not interchangeable between these models.
The gas cylinders are welded in slightly different locations. This is partly because the 11-87 has a longer magazine tube. The 11-87 also has 2 gas orifice holes. Only the 2-3/4 inch chambered version of the 1100 will have two. The magnum 1100 barrel has only one.
The 1100 can shoot only 2 3/4" or 3" shells depending on the barrel installed, and with the 1187 models with magnum barrels can shoot both 2 3/4" and 3" 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.
The Model 11-87 also has a slightly longer barrel extension. This is the part that connects directly into the receiver.