So now explain to all of us how"X" rifle will fire a 150 grain bullet faster than it will fire a 180 grain bullet but the 180 grain bullet will have more KE and momentum. Ifboth bullets are being pushed by 60,000 psi of chamber pressure how exactly does the pressure pushing the heavier bullet become more efficient than the pressure pushing the lighter bullet?

Regardless of what we want to call itthe heavier arrow will have more KE and momentum. It may not be a whole bunch but it will be there.