User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-5379655-20130128042915/@comment-5379655-20130129033231

Okay I guess I didn't really make myself clear enough the first time, so I'll go over some of it again. Frigates actually do not carry mass accelerator weapons. Until the development of the Thanix cannon, no mass accelerator weapon was small enough to be mounted on a frigate. Frigates carry GUARDIAN lasers for fighter screening as well as disruptor torpedoes. A quick check of the space ship page will verify this and that page is canon, based on the codex. In fact, the only military frigate displayed in the series is the Normandy and the SR1 didn't carry mass accelerator weapons. Every other non-dreadnought ship in the series is actually a cruiser and this can also be verified by looking at the pictures for cruisers in both the ship page and the codex. Frigates are meant for strafing runs through enemy formations, not long distance slugging. Now that that's settled I need to address a misconception you have about ship speeds. Ships only move at relativistic speeds when they engage their FTL drives, not while in combat. Therefore, VIs are not attuned to target things moving unpredictably at relativistic speeds thousands of kilometers away. While the VIs are impressive they also miss Dreadnoughts from distances of thousands of kilometers (Ship combat in the codex), not to speak of the hundreds of thousands of kilometers you are suggesting with you ultra-long range sniping. And when I said that the PDCs are hurtling through space at thousands of miles per hour, I meant that (relative to the ships shooting at them) they are on a planet and that planet is hurtling through space at those speeds. A good way to think of this is pretend that someone draws an x on one side of a baseball. Then a professional pitcher throws the fastest curveball in his life with said baseball, perfect ball rotation and everything. What you're proposing is hitting the x exactly in the center with a rifle from more than a mile away. Even with advanced computer targeting, I bet you'd have a hard time doing it.

Another way of looking at it: Reapers has the most advanced tech in the galaxy as well as the strongest mass accelerator cannons in existence in the form of their spinal guns. Even they enter or at least approach a planet before destroying cities with their cannons (Multiple codex and galaxy map entries).

Like I said before, great in theory, ridiculously hard to pull off in practice.