One thing I haven't seen mentioned is how this affects some of the older Parker's. In the hammer gun era, a lot of the 10's and 12's were chambered for brass shells. They came in two sizes, one smaller than normal and one regular chamber diameter. I just bought an 1880 10 ga set up this way for the "A" size shell with normal dimensions.These guns were ch.ambered to exactly the open shell dimension with a step down to the barrel diameter and no forcing cone at all. I did a lot of careful measuring and confirmed this. Parker was apparently rather proud of this system and stated it gave the best performance. I suspect a shell even slightly shorter than the chamber would give a significant pressure spike.