I always liked the Sterling magazines and never had any problems with them, not lightweight because they are steel. I have tested the 40 rd Sterling a few years ago and never had a problem. The Sterling magazine will fit both the AR-18 and Colt SP1 too.

my photos show the early Armalite 30rd, Sterling 30rd, M16icon 30rd and Sterling 40rdAttachment 103765Attachment 103766