The barrels are different on 3" and 2-3/4" guns. The 3" barrels have one hole, the 2-3/4" two gas holes/ports. I know what Remington says/said, but every receiver I checked weighed the same and measured the same, including the ejection port. I admit I did not measure them all, but I would estimate I checked at least 10 magnums and 20 standards. And I am not talking a bathroom scale and a ruler, either. The action springs, the ones in the stock, are different. The magnum spring is softer. The action bar sleeve/assembly is different also. If I remember right the magnum is heavier.
For about the last 20+ years I have been running a 12 gauge 1100, 2-3/4", circa 1964 vintage, with a 3" Remington "Steel Only"' barrel (which appears to be exactly like every other 3" chambered Remington 1100 barrel, including the as delivered port size) installed for waterfowl. Remington recommended this set up for 2-3/4" guns for steel shot. I did not change the spring or the sleeve. I did have to drill out the gas port a few thousandths to obtain reliable cycling, and that was as per Remington's instructions when I first got the barrel. I have used it with all kinds of 3" shells, and it will not function with anything less than very stiff 2-3/4" or 3" loads. That's okay with me, because I think it's easier on the gun/receiver. The action barely opens enough to function rather than slamming into the buffer. I bought 2 buffers years ago, and installed one, but I have only used the 3" barrel since (other guns for other game(s)), and the buffer still looks new, far different than when I ran the 2-3/4" barrels.
