The HVM Heavy Machine Gun Sentry is the cheapest, though not the most cost-effective, turret. Unfortunately, the limitation of 2 turrets (until you get level 5 mastery for turrets, which extends the limit to 3) makes it ineffective in later levels (50+), due to the far lower damage or utility output compared to other turrets, and the far stronger zombies. It is, however, good for covering you while you reload, killing medium swarms while you fight a boss, or shooting Shielders from behind.
However, this turret is highly effective against weaker medium sized swarms of zombies, and even can be used to take out Bloaters or a chunk of a boss's health. Overall, this turret is a great mid-game weapon and can mow down hordes of zombies due to its acceptable pierce and fully automatic stopping power.
The BLACK version's Damage is unknown due to it not being in the game, although the BLACK turrets are a questionable thing, in terms of the possibility of NK adding them.
As an automatic turret, HVM Heavy MG Sentry appears to be a futuristic stationary heavy machine gun with a rectangular platform. It has quite big muzzle brake (possibly due to its high caliber as for a turret). It feeds with ammo belt on the right side of the turret (similar to Z-5, which is a LMG though).