Here's an idea for a weapon... It is a gigantic ion-railgun called the Cleaver. It has infinite (or high enough that it may as well be) penetration, shearing through everything in it's path, with the exception of energy shields, which stop it in it's tracks. It would require (bear with me) A sensor for placing, and both 32 bullets and 48 plasma cells to fire. It has an insane range, able to target enemies that are as far as five kilometers. The biggest downside would be that when destroyed, it will take out a huge chunk of your ship, and start fires.
