the "black powder" bevel, thinner tapered barrel and round-butt frame all contribute to a lighter revolver when in the mountains, every oz. makes a difference
That said, I have both a 629-4 and a 629-4 Mountain Gun, both 4 inch barrels, and the difference in weight between the two is the Mountain Gun is 2 oz. lighter !
Really more "show" than "go" but it is slightly lighter and more visually pleasing to me !