Staggered box magazine, formed steel body and feed lips, polymer base plate and shoulder.
Quality is standard. Always. Detachable magazines, often a costly upgrade to other factory rifles, have been a standard feature on Browning big game rifles for decades. The AB3 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.
Keep one handy. Having an extra loaded magazine in your coat pocket while hunting is undoubtedly better than a coat pocket full of loose shells mixed in with your truck keys and that half-eaten granola bar you were saving for later.