Aye, and the GAU-8 is 20 feet long including the barrel!
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how much weight (in ammo) do you think they could fit into a tank carrying such a weapon? also where would the crew sit?
I think you're mixing apples and GAUs...or something.
If you look at an A-10, you need to realize that the GAU-8 is so big it actually extends underneath and behind the cockpit. If you attempted to mount something like that on an armored vehicle the size of the M1 you would have to remove the turrent entirely, and box in the back end, simply to be able to enclose the ammunition feed mechanism...not only is that *not* a good idea in a tank (do a web search on a Swedish tank known as the 'S tank' which tried to do something similar) as you're stuck changing your point of aim by turning the entire vehicle; it would also throw your center of gravity off... to say nothing of covering up the exhaust grills of the engine compartment.
And while we're talking about the GAU-8 there are two more items to consider. First off, the reason the automatic loading mechanism on the GAU-8 works as well as it does is due to the fact that the gun itself is *fixed*. It doesn't elevate or depress and it certainly doesn't traverse (i.e. move left to right). A completely automatic loading system on a turreted armored vehicle is a *very* bad idea. We figured that out, the old Soviet Union never did...which is why there are a bunch of former soviet armor crewman walking around with only one arm. Secondly, you also need to remember that the A-10 can fly home to get a reload, when it runs out of ammo - something your average tank in combat cannot do. Thus the rapid rate of fire of the GAU-8 would be impossible to sustain in an armored vehicle, simply because the vehicle couldn't carry enough ammunition for the sort of sustained combat operations that are normal for tanks.
As far as the coupolas go - you need to do some reading on cobham type armor (the sort of armor on M1 tanks) and what it takes to mount non-cobham materials to it. Be careful, cobham is *not* reactive armor - that's something different but there are a number of internet sites which intentionally seem to confuse the two.
I suggest you might start by reading the relevant portions of Janes Weapons Systems and Janes Fighting Vehicles, and go from there. It's good that you're thinking about ways our armored vehicles could be made better, but you might want to do a bit more research into at least the basics of what makes up a modern tank, first.