The weapon model is an American variant of the Belgian FN Minimi, namely the M249 PARA. Unlike many weapons in the game, which hold much more ammunition than their real-life counterparts would, the LMG's 200-round ammunition capacity is entirely accurate (Though it is worth noting that the weapon is modeled with a cloth-material "recce box", which only holds 100 rounds).
With its power, fire rate, and ammo count, it's very capable of clearing large crowds of zombies with ease. With Chuck's speed maxed out, one could just run and hold the fire button with this weapon equipped without much worry of zombies grabbing hold.
When battling Sgt. Boykin, he will occasionally hold his own machine gun in one hand and fire at Chuck at a rate of fire not normally possible on regular MGs obtainable by Chuck. This could also possibly be a reference to Rambo, as he is seen firing a light machine gun in this fashion.
Boykin's machine gun will not spawn if Chuck has one of his own, even just after defeating him.
There is a way to get two LMG's. First you must have done Case 3, in order to unlock the yucatan security room. Then, you must enter the Yucatan Casino with no LMG's. There should be one in the vault room, and one where you find the Zombrex.