The Browning Automatic Rifle (BAR) is an automatic rifle that saw service from World War I to the 1970's. It uses the .30-06 rifle cartridge, the same as the M1 Garand. It has three firing modes: high full auto (650 rpm), slow full auto (450 rpm), and semi-automatic. It would be wise to use the semi-auto firing mode, so as not to waste ammunition. The .30-06 rifle cartridge should be quite easy to find in most gun stores in the United States, another good thing, this particular round has enough stopping power to punch through any zombie's head.
Although the BAR was used as a light machine gun during it's years of service, it can also be used as a rifle, hence the name Browning Automatic Rifle. It is a bit heavy, and it's recoil can catch you off guard, so a recoil compensator helps. The only possible way to attach weapons modifications would be to upgrade this weapon with tactical rails for mounting sights and other accessories. An accessery such as a scope would help with long range engagements with the undead. A verticle foregrip could help the user control the recoil. A bipod helps keep the gun stable when firing while prone, but can also add a bit of weight to the already heavy rifle.
However, as most BARs can only be found in antique shops and museums, it is not a weapon that can be found in any common gun shop. The magazine capacity is small, said magazines are rare to find, the weapon is too heavy to be practical, and with no option to fit a suppressor without massive modifications, this is not a weapon recommended to be used against the undead.