With regard to reducing the wall thickness, lengthening the chamber will push the corner of the chamber/forcing cone further forward. Since the outside of most sxs guns has a taper from the breach end, this definitely reduces wall thickness. However, I haven't run across a gun with a tapered outside that was steeper than the longest forcing cone reamer I have. Therefore, the minimum wall thickness will still be the corner of the chamber/forcing cone, regardless if the cones are lengthened with my reamers. I believe someone stated that modifying cones on a British proof gun doesn't take it out of proof? If so, this would be the reason, IMO.