Chuck, 4140 is very good steel and I don't know for sure on how many barrels were ever made of this. I think most were in the high 10 series and would think the elasticity is different, and so would the tensile strength.

Regardless, thanks for looking into this, and 5" from breech is still way to close for comfort. It could have been an imperfection in the steel to cause this and an x-ray might help to determine this.

David