I am pretty sure there will be some info on the 'net about gas port sizes and short barrels with the more popular gas guns...such as the 1100 / 11-87. I have fired a 12in. (approx) 1100 many years ago that was modified by...I believe...Sage(?)...and it worked fine with heavy Buck loads (there were NO reduced recoil loads back then) I have also seen a 14" 1100 that seemed to work just fine with the same type loads, but never fired it or asked what mods may have been done on it.

I really...really...like and prefer a 14"... or thereabouts... barrel on my shotguns for serious use, and that is what I would be looking at if I was going to do a SBS with an auto. If you want to stay with standard, legal barrel lengths, with no registration required, most gas guns will run fine at 18-20in. lengths using FULL LOADS...remember, "Tactical Loads" are NOT the same as the older "full" loaded shotshells!  Less than that...especially at 14" or less...you will have to alter gas port sizes, I think. (My only SBS is an 870 so I am not an expert here by any means!)