The forends are an issue, as is the method of the retention method for the magazine tube cap. The 1187 (and late model 1100s) use an internally toothed cap with a toothed plastic plug in the end of the mag tube. Most other 1100s use a detent ball in the face of the forend. (One way around this is to use an o-ring under the cap to retain it. Less than perfect, but reports are it works.) On top of the that, there is a dimensional difference in the "barrel hangers" (properly called the gas chambered). The 1187 uses a sheetmetal exhaust shield for the auxillary gas ports (which the trap barrels lack) but regardless the forend may still need it to be properly spaced. Then there's the issue of the o-ring inside the gas chamber. The groove on the mag tube are in a different position. Generally this works OK anyway, but, some have found they could not get a proper seal.
Late model 1100s use the 1187 bolt with the wider extractor. So a late model 1100 barrel may be OK. An earlier 1100 barrel may have a problem. This can be fixed by carefully filing both sides of the extractor slot wider on the barrel.