The BAR, Rem 7400, AR-15, AR-10, M1A and many others are gas operated semi-automatic rifles. They were all designed to have the gas (created by the burning gun powder) extract, eject and insert new rounds. To do so consistently, they were all designed to operate within certain pressure ranges. The Hornady Light-Mag cartridges and the Federal HE cartridges all produce pressure outside of the designed tolerances of semi-auto rifles. If used, you would probably notice violent extractions and jams or misfeeds. Eventually, it can cause the weapon to basically destroy itself. Now the BAR is one of the most sturdy semi-auto actions made, but the excess gas would eventually destroy the rifle. And most people prefer not to be holding something next to their head and have it come apart, violently, in many pieces. Now some will note that the BAR is chambered in 300 win mag and 338 win mag, and they operate at high pressures. The difference is, those particular models are tuned to handle the higher pressures, basically by having smaller ports to allow less gas through. I hope this answers your question.
