The biggest gun we can theoretically build is 2000 feet long. But the initial acceleration would kill any human being. Then the gun experts have an idea! Water!
We can add a layer of water between the explosive and the capsule. When the explosive goes off, the initial split second shock compresses the water, like a spring. All of the force still reaches the capsule, but the force is spread out, and shouldn't kill everybody inside.
So the gun should work! But how can we pay for it? The iron alone would cost millions of dollars, and that's before we factor in labour and all the other expenses!