Awarded Best Picture (Feature), 2016 Comicpalooza Houston, Tx.
Johnny Strong used a similar rifle, an M14 in the movie Black Hawk Down
The movie was also released in a interactive virtual reality experience.
A plan by the survivors in the movie was to make it to the southern tip of the Baja peninsula because the zombies could not walk that far in under one day. At the beginning of the movie, Rourke (Johnny Strong) comes across a zombie girl holed up at an abandoned gas station in the middle of the desert inside of a freezer just after sunrise. The scene suggests it would have taken too long for the zombie to reach another city in less than a day.
The protagonist (Rourke) carries a SOCOM16 variant of the M1A, it uses 7.62x51mm (or .308WIN) which is a very common round used for hunting and would be found in anyplace that carried ammo, as well as likely picked up from other weapons encountered. As it was the go-to military round prior to the adoption of the 5.56x45mm used by the M16/M4/etc.