Anything is feasible with time and imagination.
Problem with a mini gun, is you need an external power supply. Usually 24-28 volts and 58 amps to get it running.
So, there's 3 batteries you'll need in your 4Runner.
Then, there's the immense firepower of that weapon. 3-4k rounds per minute. In order to have any fun, you'd have to fill the inside of the 4Runner with ammo. Even then you'd only have maybe 5 minutes of firing time. You'd have to link all the ammo up, so there's some time wasted, as well as trying to figure out how and where to send the brass and links, seeing as he they fall through the bottom of the weapon system.
Like I said, it can be done, just gotta overcome some obstacles first.
A mini gun would be fantastic on a 4Runner. If like to have a system that I can mount different weapons too. A 240B, 249 SAW, M2, ect.
I like the power of the M2. Rounds are expensive, but you can get everything from FMJ, to armor piercing, to incendiary.
My vote would be to mount a M2 on top of my runner.
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"We are experts in the application of violence."
-LtCol Christian Cabannis, Camp Letherneck, Afghanistan, 2009. The "Summer of Decision."