I think you got that backwards. The early models did have the detent in the magazine ring, and the newer ones use the "green monster" saw toothed plastic thing inside the mag tube. A 3/32" 'O' ring will hold the mag cap on just fine. All the barrels will fit. If you have a 2-3/4" receiver gun, 3" hulls may not eject properly even if fired from a 3" chambered barrel. Ejector is different.
What could have happened... did.