clandestine
AR-15 Savant
Then they must not like to update their website. This looks like the closest thing they have posted to what you're describing, but it has what looks like an M-14 mag in it.
http://www.armalite.com/ItemForm.as...ategory=4052a69e-f815-4c59-a4dd-6466fd4c4ab8#
I may have used the wrong description but they make it. Its the ones shown with pmags in them. The standard AR10 pattern mag is steel.
All of these take .308 Plans which are DPMS/SR25 pattern mags.
http://www.armalite.com/Categories.aspx?Category=a1bbbf86-b2ed-495f-be70-47ca6b86de76