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human
[Metal Gear Solid] If the whole purpose was to launch nukes with a rail gun anywhere in the world, then with bother building an expensive bipetal tank-mecha (REX), when they could have simply mounted the rail gun on a ship?

I just got done watching the entire MSGS cinematic on youtube because I'm to lazy to emulate the game or get a PS1.

The whole point behind constructing Metal Gear REX was to use it's rail gun to launch a nuke from anywhere in the world, to anywhere in the world, without use of a missile, making the nuke invisible.

I also get that REX can walk anywhere and thus make himself within range of any country in the world. 

BUT

Can't a ship like a Navy Destroyer basically do the same thing while not being limited by the ocean? (No game addresses how REX is supposed to cross an ocean).

It seems the issue of the game was not REX itself, but simply the Rail Gun mounted on it.

Metal Gear 4 makes it clear REX was not designed as a frontline assault weapon, but more of a long-range mobile weapons plataforn.


Wouldn't it make more sense, from the logical and financial aspect, for DARPA to simply invest in perfecting the rail gun platform itself, and then simply mount it on ship, or heck, even tanks, (judging by the size of it in MSG1)?

RAY was supposed to be a all around infantry and combat unit design to overpower enemy mechs and mobile infantry, so I get it's purpose.

I don't get the purpose of REX, who's designed to fight long-range.

Why even bother building a gigantic bipedal highly futuristic and expensive mech that will only serve as a glorified artillery piece?  Shit, a rail gun mounted tank would to the same thing while being less of a target.

I know it breaks the suspension of disbelief, but isn't this how it goes IRL? Military prefers what's simple and works instead of what big, flashy, destructive but super expensive?
gpt
A big issue with the MG world is that they have a different chain of historical events, most critically the shenanigans of Big Boss, Zero, and the other players of their strange game of global control. By the time of REX, the world has already been menaced by multiple nuclear-equipped vehicles of assorted sizes, configurations and threat levels, and there's a meme that whoever manages to perfect and control such a weapon system will have an edge in geopolitics. No one stops and says "wouldn't it be a better idea to put this on a conventional ship" because they think quadruped or biped mechs are the current hotness, while conventional surface ships aren't. Conventional ships are easy to track via satellite assets or by following hunter-killer subs, while REX could disappear in any sufficiently large building or tunnel, or be dismantled and smuggled in containers, or otherwise shuffled in a way that impedes conventional tracking, before letting rip with a ICBM tier shot that hits with minimal warning from unpredictable vectors.