Keep in mind rotational inertia. You can't just accelerate in any direction that you want at a moment's notice.
You want a kinetic round with a diameter of 1m? Ummmm... Let's say it's a cylinder 1m long. That's a volume of 3.14 m^3. If it's only the density of water, that's 3.4 million cm^3, or 3,400 kg. If it's steel, you multiply by about 7.8, for 26,520 kg. 26 metric tons ... That's just a ridiculous mass. Over how far a distance are you going to accelerate this thing?
If you want to get to 10,000m/s with a barrel length of 100m, you need to accelerate it at 100G (give or take). That's not overwhelming, but that's not a turret-mounted weapon and you're going to need an immensely strong structure. After all, the force is 26,520,000 N, if I got all the decimal points in the right places ... If you want a shorter barrel, you need to increase the acceleration, so a lot more newtons. A 20m barrel pushes it up to about 125,000,000 N. Ouch.
Converting to GJ is left as an exercise for the reader.