Nosferatu - the gap between the barrel and mag tube is the same on 1187's and 870's IF the 870 is a D serial #. They recently changed it. If you have an older 870, the gap is a little bigger.
I didn't know any of SAU was using 590A1's. I thought all of the shotguns were 870's.
All of the 870's we use at the academy get minimal cleaning. I've seen them get a little "sticky", but it's nothing a little oil doesn't cure. I've never seen one fail. And they get a lot of use.
I like my 870 and I have little experience with the 590A1. Is there anything about it's design that differs drastically from the 870 in either fit or function that makes it a "better" shotgun?