Venus or Mars for terraforming and colonisation?

[halfeye]

The potato's just not going to stay together.

It's not, but it will exist for a time, and in that time, due to its extreme speed, it will travel a very long way.

Rail guns exist, the US navy has even had one mounted on a ship for some time. However, projectile physics work the same regardless of how you accelerate the projectile to that speed.

Projectiles do work the same given the same speed. Sabot rounds are much slower than rail gun rounds would be. Last I knew rail guns were still experimental, if they aren't being fitted as standard in new vessels, I'd say they're still in the prototype stage of development.

For things where a lot of penetration is desired, you generally use an extremely dense core for this reason. Tungsten, DU. High speed, dense core, longer round, those are the hallmarks of extremely high penetration weapons.

That's tanks, but it wasn't WW1/2 battleships, they achieved penetration with very heavy shells, long yes, but wide too.