Modifying a M14/M1A magazine for use in a AR-10

I give some people ideas I really shouldn't, a friend was looking for spare mags for his AR-10 and I thought "hey I can make some out of Import M14 mags for you" never having tried it before....My last trip to Knob creek had yielded  a small lot of M14 korean import magazines new in the wrap I thought perfect for the job. The first thing I realized is the mags are VERY close....but the AR-10 mag costs $45+ or so new and my Korean imports were under $10. 

AR-10 magazine on the left, Korean import M14 magazine on the right.

First thing was to take both mags apart and see if there were any limiting factors involved in the modification.
The spine of the two mags showed the biggest difference. The tab on the import magazine serves as the catch for the M14 magazine release, there is no such tab on the AR-10 magazine meaning you could not use the AR-10 readily in a M14. Also note the tab on the follower of the AR-10 magazine, this travels in the channel formed in the spine of the magazine, this serves as your bolt hold open after the last round has been fired.  The M14 magazine follower does not have this feature nor could it be easily modified to work in this way. After the modification the follower itself stops the bolt, much like that on some AK-47 mags, when you pull the magazine out the bolt goes home. I realized these magazines would serve well at the range but I would not want to rely on them for serious social encounters without making a modified follower. I had a idea for a spring loaded tab but it would have been more work than what was needed for this modification.

The difference in the spine and followers can be seen here. The tab on the AR-10 follower left is your bolt hold open, our mags would not have this yet the bolt will stop on the follower itself much like some AK-47 magazines.

Here I am pointing out a ridge formed in the magazine feed lips, it was enough to cause  a little hangup if you try to load these into the magazine too fast.

 I don't have any pictures of the modification for the magazine latch hole, pretty much if you have an original magazine you need to follow that same angle and size, GO SLOW, if you know when not to use a dremel this is one of those times, you may rough out the hole with a dremel but after you get to about 75% you use a square file to true it up. If you do a bunch of these its worth it to make a cutter for that particular size hole.

Here I'm using a woodruff key cutter to make the relief needed in the magazine body for the bolt hold open.

Its pretty easy to modify these cheaper magazines to work in a AR-10, Just go slow and check your work often. Be sure not to crank these magazines into a vice really tight as you will squish them EASILY.