Hi, i don´t think it was used in WW2.
If you look at one of the last pictures of the AR-10 at gunpics.net you will see "cal. 7.62 NATO" on the AR-10. In WW2 there was no NATO!!!!!
I think it was one of the first AR-10´s ever made in the 50´s. And the AR system was originally made for the 7.62 round and later they build the one in 5.56mm.
Joker is correct. NATO wasn't around and I assure you there were not AR's in WW2 - the closest thing was the German Sturmgewehr (Stg) and thus, the great grandaddy of all assault rifles as we know them today.