So I went out shooting my new-to-me (mint used) AR-10 after mounting a decent scope to it (Nikon Monarch Gold 2.5-10x50mm) with some Warne high mounts and a Harris Bipod. The rifle functioned and shot perfectly, except on a few occasions the last round in the Armalite factory 10 round magazine didn't want to feed. Could this be a weak spring or does the magazine just need to be lubed or is this a common problem with these 'hybrid' magazines...
Take your mags apart.
Take a sharp knife and "melt" all sharp edges of the follower.
Rub, polish, whatever you want to call it, the inside of the mag with 3M pad material. Get under the feedlips real well.
Reassemble with the mag spring's second coil locking the bolt stop plunger in place and the first mag coil pushing up underneath the front of the follower.
Come back with a range report.
The problem is pretty common because of the stupid plunger on the back of the mag. Try working the mag in with a ruler, just compress the spring a bunch of times. I had that problem with two of my factory mags, but it seemed to work itself out. Armalite has redesigned the mags to try to eliminate the problems, but I haven't tried any of the new mags myself.
As far as extra mags, you can buy them from Armalite, or check the EE. Several members sell converted mags for pretty good prices. I bought two that have worked perfectly since day one. I have two 10rd factory, two 20rd factory, and two 20rd converted mags from the EE, and not one of them has had any feeding problems since the rifle was new.