Machine Gun turrets (abbreviated MG) are the most balanced turret, and used by nearly everyone. They continuously fire at enemies using a machine gun, hence the name. However, they have relatively low range, being second to Magnifiers. Their fast firing allows great damage against swarms of un-armored enemies. Upgrading increases damage and range.
Paired with Zapper Turrets and Scouters, you can create a nice stup, complete with long range, great slowing effects and fast damage. Unfortunately, MG turrets are now very ineffective against larger enemies (unlike when the game was first released), so most players have switched away from them. However, if just beginning, a couple MG turrets or more are highly reccomended.
The Machine Gun Turret costs 200$ to build. The upgrades cost $300, $300, $400, $500, and finally $2000.