Will my 20 gauge Remingtom Model 11 shoot 3 In shells. I believe it was made in the 1950,s and has less than 4 box's of shells shot thru it. Many sites say all 20 gauge Remington Model 11 will shoot 3 inch shells.? any truth to that? Thanks in advance.

It probably will shoot, but I doubt that the chamber is for a 3 in shell. Don't forget the lips of the hull still have to open up and any restriction will cause pressure to rise. You can get a chamber gauge that will show the length. Then you have the ejection port length and possibly the feed latch timing.
I would stay 2 3/4”, I had a nice 20 ga solid rib model 11 I should have kept it.

It should say Mangum on the receiver side and if so will probably shoot 3.....also on the barrel it shoot state shell length of 2 3/4 or 3".I do not know when 3" shells were introduced, but in the 1950's it may have been just 2 3/4 shells.

Remington model 11’s were never made in magnum versions or marked as such to the best of my knowledge
Model 1100 were but thats a different platform