“So what is your reason for wanting it?”
Everyone knows you need something like that as a hedge against alien (from outer space) invasion. An AR-15 might be ok against little green men, but when confronted by a giant BEM (Bug Eyed Monster) you want a sure one shot stop.
Go for the .458 SOCOM variant of the AR-15 instead. Same bullet, more energy, lighter platform and easier to customize as far as optics, etc.