Detachable magazines, often a costly upgrade to other factory rifles, have been a standard feature on Browning big game rifles for decades. The AB3 rifle is no exception, featuring an extremely tough steel and polymer box magazine that protrudes just enough from the stock to allow you a tactile reference if a mag is inserted in the rifle.
Having an extra loaded magazine in your bag or pocket while hunting is undoubtedly better than a pocket full of loose shells mixed in with your truck keys and that half-eaten granola bar you were saving for later.