SteveSatch:
One thing about factory Remington magazine extensions (+2s and +3s) is that they do come with a barrel band to attach a sling swivel mount to. However, Wilson has a sling loop mount that is available for the 870 and 1100/1187 that fits between the locking collar of the Remington factory magazine extension and the barrel band.
It is simple to install, cheap (about ten dollars), available for side or bottom sling mount configurations, and shortens the distance between the sling mounts, which is more comfortable to carry on the shoulder since there is less weight of the shotgun riding below your waist. I bought one from Brownells a couple of years ago for my 1187 with a +3 factory extension and found it a 100% improvement over the barrel band sling mount. I also had checked with Wilson over the compatibility of the sling mount with the factory extension beforehand.