As has been pointed out the barrel extension ( the part that slides into the action) on a 1100 is much longer than a 870 but also the portion of the part that slides over the magazine tube is also much larger on the 1100 as well as having the gas ports as mentioned above. The 870 is probably about 3/4 " while the 1100 is about 2-2.5". If you could post pics it would be much easier to ID it for you.
