There isn't a standard AR-10 mag, but the KAC SR-25 / PMag LR is getting close to being the defacto standard.
There's the original Armalite AR-10 pattern which was used on the Hollywood, CA and Belgian built real AR-10 rifles.
Then, a few decades later, Eagle Arms of Geneseo, IL bought the rights to the Armalite name, and the rights to the AR-10 name. They switched over to a modified M14 mag for their AR-10.
Knights Armament, DPMS, and LMT were the first three of the big companies to use the original AR-10 pattern. Remington, Bushmaster, S&W, and now Ruger have followed suit. These (and probably a few others) are the rifles that the PMAG LR is compatible with. To further confuse things, Eagle Arms began offering the Armalite AR-10A which also accepts PMags (and KAC and DPMS mags).
CMMG and SI defense used to offer .308 AR type rifles that used HK G3 / HK 91 mags, but they've switched over to the KAC / PMag pattern as well.
Rock River Arms' LAR-8 uses inch pattern FAL mags. RRA bought this design from Bushmaster when Cerberus Capital / Freedom Group bought Bushmaster, and switched Bushmaster over to the DPMS / KAC receiver and mag pattern. There were a very few FAL mag Bushmasters shipped before the CC / FG buyout.