FYI...I have both an 1100 and 1187. The gas pistons in each are identical. Nothing has been changed since purchase, and the 1187 owners manual shows two piece piston. Thefore, I guess I have the "older" 1187? The two guns tear down exactally the same, only differance between the two is the 3" chamber on the 1187. I shoot 3dram equivilant loads with 1 1/8 oz. shot all day long at skeet and clays. I've never had a problem from my 1187 with this load, although I have heard that a 1 once load might cause a problem.
The gas system being the same in both guns does confuse me though. I intentionally bought a 2 3/4 inch chambered 1100 because I had friends that had a 3" 1100 that wouldn't shoot light loads. They always had to shoot high brass, even at doves! I thought that was the reason for "upgrading" the 1100 to 1187 with a different gas syatem to accomadate both light and heavy loads. Yet, I see no difference in the guts of my 1187 as compared to that of my 1100. All I know is that both guns shoot light target loads without a problem.
One last thought: I have also read that things were changed by Remington because too many people were putting 3" 1100 barrels on their 2 3/4" 1100 recievers, causing a real safety issue, and that the gas system had nothing to do with the "upgrade". If true, this would explain why the 1100 and 1187 barrels are not interchangeable. Maybe we could get a Remington rep to reply to this thread and end this mystery once and for all.