Momentum (vector) vs kinetic energy. mv vs 1/2mv^^2 (scalar)
but if you rely on momentum you end up questioning chrono readings - we shoot with the butt held in the shoulder which reduces any reverse movement of the rifle but when using the chrono everyone just holds the rifle loosely (relatively) which lets the rifle move backwards. Upshot is that the pellet velocity should be lower over the chrono than it would be when holding seated, etc.
Pellets may splat but they almost always bounce off the target (easily seen) so they do not entirely transfer their momentum to the target.
You need to use something you can measure independently of how the energy is transferred.
Anyway - something like that and I really did't want to start a chrono thread :-)
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