M-14 mags: $60/each, and climbing.
FN-FAL mags: $4/each, and stagnating.
Converting an M-14 mag for AR-10 service is a risky and expensive gamble.
I don't doubt it can be done, but it may not be as simple as just reworking a magazine catch. The feed lips of the M14 magazine are very short, less than half the length of the magazine, as was necessary for feeding with the M14s Garand-derived turnbolt action.
That could well mean that not only would the rifle's receiver magazine well dimensions need reworking, but also the internal bolt guideways, and either a rearrangement of the front magazine pivot pin or modification of the magazines' front pivot lug, necessating a more complicated rework and removal of the seperately fabricated and welded-on front pivot lug of the inch-pattern FAL magazines. Then there's the matter of the bolt catch, as tripped by the rising follower of an empty magazine, which works a bit differently on a FAL.
I think it's a good deal easier to just go with the FAL, which also can be transformed into a handier carbine-length shorty without altering the rifle's sights.
It's certainly possible. But it's not without some problems. Better, I think, to work up a new design altogether, as per the Robinson Arms Model 96 *Expeditionary Rifle* in .223....
-archy-/-