It's a segmented high tensile steel tube with an inner steel tube filed with sand. It has a 4140 steel disc at the bottom over and and a 4140 steel sand pot over that - not like any boolit will make it that far! Actually, I used to use water soaked rags as a catch medium to test for boolit performance. Then the boolits did make it into the sand pot.
Like you say, it can be hard to evaluate a recovered boolit due to impact damage.