The 551A1 is a really nice rifle. As the whole is more than the sum of its parts, I would encourage you to look it over really well.
It uses proprietary magazines, which rock in like the M14 or G3, but are inherently ambidextrous for the magazine release is located in front of the trigger guard.
Has cold hammer forged, nitrided, barrel, 1 in 7 twist, unique flash hider. Mainspring over oprod/piston. Two position gas system. Pic rail for optics, and foldable "swiss style" stock.
It really is a neat weapon.
As another contributor has said here, upper receiver is the serialized part, so you can change out the lower to one which accepts AR15 mags if that suits you.