AA Superhandicap and Sporting clays 7 1/2's cycle my 1100 magnums (single gas port).
They work flawlessly in dual port field grade 1100's too.
(ports a little smaller than those of the skeet barrels).
On steel shot loads, from the Remington Arms Society guys, they say that if running light loads that opening the ports a letter drill size or two (max) may be required.
I'm not sure what port dim is used on 1100 Steel shot magnum barrels, and they couldn't tell me.
My 28" VR Remchoke 1100 Steel shot magnum barrel will not cycle steel shot dove loads on reg or magnum receivers (there they are supposedly the same but carriers are of different weight to lessen battering possibility- both bolts have same buffer/recoil spring).
And yes, my bbl is a 28" Steel shot 1100 one. Supposedly Big Green didn't offer 28" as an accessory bbl, only 26 and 30, but the proof marks are 1100 and not 1187, and the warning on the bbl wouldn't be needed on an 1187, and the choke tube is a remchoke and the bbl flares to accept the choke tube like all others.
Read: it is a legit 28", but if not sold as an accessory bbl, must have come from a limited run of complete guns and become divorced somewhere along the way.
It cycles the AA Superhandicap and Sporting clays lead lods fine, has on 4 different 1100's.