The Barrett and PTRD could both damage soft skinned vehicles, lightly armored vehicles, and antique tanks.
Heavy machine gun originally referred to machine guns with a fixed mount and a design that heavily emphasized sustained fire over portability and were chambered for service rifle cartridges. This definition is not the most modern, and weapons of this type could sensibly be reclassified as medium machine guns (being chambered for service rifle cartridges) or heavy machine guns (being difficult to transport and impractical for use without a mount.)
Medium machine guns are chambered in service rifle cartridges, and sacrifice some of the sustained fire capability of the earlier heavy machine guns for relative mobility.
Light machine guns further sacrifice their sustained fire capability to be manportable.
Automatic rifles are of dubious utility into sustained fire roles, but are portable enough to be used while on the move. (There have only ever been a few automatic rifle designs, so it may make sense to group them with battle rifles, but it doesn’t make as much sense to classify a variant of the BAR as a light machine gun, simply because it has a bipod (which in the Unturned attachment system, would be quite effortlessly removed.)
General purpose machine guns were designed as more or less light machine guns, which have the ability to be mounted for use in or against aircraft (though only effective against low, slow, and soft aircraft,) or to be used in place of medium machine guns (but lacking in the stopping power and sustainability of either type of heavy machine gun.) General purpose machine guns have largely taken the place of both light machine guns and medium machine guns, but would generally have more in common with light machine guns, simply with the ability to be used in a mounted role.
Section automatic weapons, squad automatic weapons, light support weapons, whatever-the-fuck-you-wanna-callems were designed to be more interchangeable with the assault rifles that have become the primary small arms of most militaries, some are only share the same intermediate cartridge as assault rifles, some have interchangeable magazines with assault rifles, and a few are actually just assault rifles with attachments and components that aid in sustained fire roles. Because they are lighter than GPMGs and the remaining LMGs pressed into service as GPMGs, and are issued similarly to how light machine guns were issued, these weapons are sometimes called light machine guns, despite GPMGs being technically more similar to the original LMGs. Those that are just variants of assault rifles should be considered assault rifles, while those that are weapons designed specifically for this role (like the FN Minimi, RPD, and RPK families) should be classified as a separate group from the Light machine guns (which are far more similar to GPMGs) and assault rifles (which despite being able to be modified to fill this role, are generally quite different.)
I suppose one of the main points I’ve been trying to make is that weapons that can be modified to fill a certain role shouldn’t necessarily be put in the same category as weapons designed to fill that role. (Carbines, while filling a specific role for an army, don’t have any meaningful distinctions from their full length counterparts that are inherent to the weapon rather than just an attachment or component. A BAR, despite being pressed into the LMG role by the U.S. government, does not have any imherent characteristics that make it an LMG. An SAW, despite filling the same tactical role as an LMG, has its own distinct inherent technical qualities which distinguish it from an LMG. An antimateriel rifle, while not able to fill the antitank role that antitank rifles once filled, (because modern tanks are much more easily dealt with by explosives than by rifles,) is still technically very similar (in size, weight, cartridge, and armor penetration.)
P.S. when looking for information about firearms, you might want to look somewhere further right than Google, or atleast scroll further down than the first result.