I have an early AR10, which uses modified M14 magazines. It made perfect sense at the time.
It was during the damned assault weapon ban, when no new mags over 10 rds could be sold, anywhere.
So, they had to decide what mag to build the gun for.... it had to be something that already existed in
large quantities, was affordable and known to be a high quality magazine. At that time M14 new mags
were $5, and the conversion, without the funky hold open pin which is INSIDE the follower, is very simple,
cut a window in the mag. The other mag out there were the HK G3/91 mags and the FN-FAL mags. The
HK mag has a big thick addon which requires that the mag well be cut very differently on the bottom,
and the FAL mags were pretty rare over hear, come in two not entirely compatible formats and need to
rock into place, where the AR10 mags to straight in. HK G3/91 mags have been super cheap at times. I
bought about 50 of them when they were $3 each, no typo! Nearly new, high quality mags. Two lifetime's
supply for the cost of about 5 of most other mags.
By the time the other folks decided to join the party, mags were freed from the Federal limits and anyone
could make anything they wanted and the M1A folks had been steadily burning through the surplus stocks
of M14 mags and suddently the "cheap, readily available mags" weren't cheap or readily available.
IMO, good choice at the time, but messed up by politics.
If you have the early AR10s, Checkmate makes high quality, real milspec mags, used by the US Army in their
M14s they reissued for the sandbox wars. If you have a friend with a mill or are good with a file, you can
cut the hole in the side and file it down to the correct bottom edge dimensions pretty easily. Nothing else needs to
be done, if you give up the last round lock back feature.
Bill