No problem [@Circ], I would be glad to elaborate. What I had in mind was a very small, miniature rail gun the size of one of Antioch's mechanical arms. It would take some time for his arm to transform into the weapon itself, and then more time further to energize the magnets within, making the weapon less than ideal for use in the heat of battle. 

Instead, it would more be used when there would be time to line up a long range, precision shot. As far as actual energy output in megajoules, I'm not too certain, but in terms of Antioch's power supply it would consume a whopping 10% per shot. This would leave him with only 5 shots maximum from full energy, before he would need to power it down to maintain enough energy to run his systems. The projectile itself would be only about the size of a baseball.

A practical use for the weapon, for example, would be to snipe enemy fighter ships from the top of a mountain when in relative safety. I would put the weapon's maximum effective range at around 8km, beyond which he simply cannot calculate an accurate shot on a moving target. 

Advantages:
-Long range
-Powerful
-Precise

Disadvantages:
-High energy consumption
-No blast radius
-Must be stationary when firing
-Long preparation phase

If you need any other details or feel I missed anything, please let me know. I really appreciate your review of my character!