AR-10 suddenly stops feeding, even first shot from magazine

[DwayneZ]

Did you happen to save the brass from the 1st 12 that did feed ok? Any odd marks on them?
What about jammed cases, what do case markings look like? Look closely all around the case from bottom to top.

Also take bore scope (any scope) and inspect the throat and chamber area for any odd things.

[bfoosh006]

What specific brand mags did you try ?

What specific brand ammo ?

How many rounds fired thru the problem rifle ? Lubed well ?

And any marks on the brass ? That will help us figuring it out.