Twenty gauge 1100 guns came in 3 different configurations. First was the full size frame guns (12 gauge frame). They were available in 2 3/4 and 3 inch. The serial number ends with X or N. Then came the LW guns. They were also available in 2 3/4 and 3 inch. They have short tangs, the part that goes into the receiver, 1 3/8 long. These serial numbers end in K or U. Lastly come the LT guns, also available as 2 3/4 and 3 inch. These have 3 3/8 long tangs.
I have a full size frame and LT. The full size gun is a left hand that my oldest daughter uses. The LT is my youngest daughters gun. Both are 2 3/4 inch guns. The full size frame guns are sought after by skeet shooters for a little extra weight. The LW guns are less desirable as they were a little less reliable. The LT has never given us a lick of trouble and runs as smooth as silk.
As far as converting a 3 inch gun to shoot lighter 2 3/4 inch loads, it can be done. I have a 12 gauge that is converted. One difference between 2 3/4 and 3 inch guns that most people don't realize is the sleeve that is driven up and down the magazine tube is heavier on the 3 inch guns. So you might have to fiddle with the extra gas port size. Putting that extra gas port in is not a straight forward job. It needs to be done by someone that knows what they are doing. The gas ports are put in at a compound angle and needs to be put in at the right place. The conversion on my gun uses a tapped hole (2-56un) and set screw to seal off the extra gas port when 3 inch loads are used which is a bit of a pain. The conversion was done by Stan Baker out on the west coast. He passed several years ago. The 11-87 was brought out to eliminate the conversion dance. They had a fair number of issues when they first came out. It was generally felt that Remington took a great gun, the original 1100, made it a pound heavier and less reliable and called it the 11-87. There were a lot of issues with them on the Sporting Clays courses and they fell out of favor rather quickly to be replaced by the Beretta's. I have not heard of many issues now but then again they seem to be relegated to field work for the most part where they must be working well enough.
As far as steel shot is concerned, I would not use a fixed choke barrel. A barrel with Rem Chokes and an extended choke of modified or less using no. 4's would be ok in my opinion. Choke tubes are a lot cheaper to replace than barrels.
