This is the holy grail of the mecha gunnery skill class. Max range of 144 lets you shoot anywhere onto the map right from your starting position! Armor piercing to boot!
Your late game config should consist of multiple phase canons letting you chain fire them at targets across the map. Found in the Radcliff (two cannons!). To obtain them, either shoot off its head or purchase a Radcliff from a shop. Some shops also carry Phase Cannons separately, which are well worth the price.
It's an even debate on whether this is better than Breaker Cannons or not. You'll be able to fire clear across the map, snipe off heads to get near-mint mecha, and armor isn't a major issue. The drawback is that it does less damage, but if most of your enemies are less durable than, say, the Savin, this is often an academic issue. The incredible range on this gun edges out the Breaker Cannon if you have enough Phase Cannons and enough Initiative to fire many, many shots before the enemy is in range; but if your mecha is fast enough, the time spent getting into a Breaker Cannon's range is minimal.
A curious note from the Shield wiki page:
The phase-effect is somewhat analogous to projected force technology, so future researchers might figure it out some day.