Of course, but having equal momentum at a given moment does not mean that two different objects will still be equal in their momentum at a later point in time. The shape and density of the objects affects that. Even aluminum - or styrene foam- have the same momentum for a given object weight and speed. That doesn't mean they will all be carrying the same momentum after some distance of travel.
Less dense objects will always have momentum issues which denser ones don't have. While the hardness and toughness of copper, and the design and construction of the X bullet design combine to mask that problem within the "window of performance," it still is a physical limitation. I happen to believe, based on what I saw and attempted to communicate, that that copper slug no longer carried enough speed, momentum, power, or what-have-you, that it needed either to penetrate well or to expand. That was why I had issues with the use of the similarly constructed 150 TTSX in 30 caliber for that distance and target. I could be wrong, but until I see some evidence to the contrary - which no one has offered in that thread or elsewhere that I have seen- I am still going to stand with erring toward caution. It has nothing at all to do with my saying TSX or TTSXs are not good in general. That is neither intended nor implied.
Sometimes, the air you 'let in'matters less than the air you 'let out'.
