OK

I am a bit of a gun person and Vietnam vet. The M2 and M134 were in use during Vietnam, and ever since.

SAW - a smaller machine gun that is in 5.56 NATO caliber (basically a .22 on steroids)

M249 (and M240) a machine gun in 7.62 NATO caliber (.30 caliber in US terms-a good size for deer and other medium size animals in a hunting rifle)

Ma Deuce - Browning M2 machine gun in .50 caliber - a much bigger cartridge and a bullet that is 1/2 inch across and over 1 inch long

M134 Mini Gun, a six barreled electric powered Gatling gun (the barrels rotate with each one firing one shot per rotation) with a control for rate of fire from 2000 shots per minute up to 6000 shots per minute, it fires the 7.62 NATO like the M249, but much faster, the M249 is "only" 650-850 rounds per minute depending on the ammunition used and how it is adjusted.

Basically they are all for when you have a lot of things that you think need to be shot in a short period of time.
And except in a few places they are illegal as they can get for private ownership. And in those places you have to get licenses and pay special taxes along with a background check so detailed they probably read your great grandparent's kindergarten report cards.