willymc
Member
My brother sent me a .300 blackout upper that he picked up as part of a build. He wanted me to check it out while he was waiting for his lower. Eventually I was going to load some ammo for him. I took it to the range today, mounted on my S&W M&P 15 lower. It loved 125gr. Nosler green tips. It wouldn't function with 147gr. fmj or 150gr. fmj.. It would fire, eject, but not pick up another round and/or fire, not eject, and pick up another round and jam it. the 147's were factory and the 150's were my reloads. My .300 blackout upper eats both those rounds like candy with no problems. Any ideas why my brothers upper won't function with the heavier bullets.