The Primary weapons group are weapons which are fired constantly with the press of the Mouse 1 (left click) key by default, being more about rapid firing and filling up the screen with your shots than heavy damage. You can fire both energy and ballistic primaries at once as long as you have both equipped.
Primary Ballistic Weapons
The primary ballistic weapons, namely the Minigun, is the basic weapon type that you start every new campaign with off the bat. Primary Ballistics are the only weapons that do not get disabled when your tank is hit by the Thunderbolt weapon wielded by Lightning Tanks. Your choice between these weapons and how effective they are for you can be dependent on what works best for you to some degree.
Your basic, starting weapon, the Minigun is always useful, with a constant rate of fire it essentially is instant damage against whatever target is directly in-front of you at the time. Of the two weapons, the Miniguns are most effective against faster moving, lightly armored targets that can dodge the slightly less consistent Flak Cannon blasts.
The Flak Cannon, while having essentially the same damage over time as the Minigun, does more condensed bursts of damage with a significantly slower rate of fire. For this reason, the Flak Cannon is effective when used against the more heavily armored targets as highly mobile targets can cause you problems with this weapon. Lightning Tanks, while highly mobile, can still be stopped pretty nicely with this weapon due to their tendency to charge you head-on.
Primary Energy Weapons
The Primary Energy Weapons take up a separate slot from their ballistic counterparts, focusing on more varied situation based uses. Although fired constantly along with your Ballistic Weapon, your Primary Energy Weapons can be disabled temporarily by the blast from a Lightning Tank. In addition, you do not begin a new campaign with one of these weapons equipped and must first acquire and leave the level with one to take it into the next mission.
The most abundant variety of the Primary Energy Weapons, this gun fires blue plasma bolts similar to those fired by Light and Medium tanks, although at a faster rate. Due to the projectiles slow travel speed as they travel in a single stream where your guns are pointing, this weapon is more effective against stationary or slow targets unless used in a right to left sweeping motion. Closer ranges will also make up for this minor drawback.
The Storm Spread energy weapon takes the blue plasma bolts and fires them in a fanning arc in-front of your tank, doing less damage to targets directly in-front of you but seeming to fire more bolts overall. This makes the Storm Spread an excellent weapon to weaken large groups of light enemies however lacking against Turrets or Heavy Tanks.
The Storm Shield is more of a defensive tool than a weapon, spawning several plasma bolts that perpetually circle your ship while the gun is firing. While these bolts will damage any enemy unit that happens to get close enough to you, what’s more important is their ability to soak up enemy shots. Don’t expect this to be some sort of invincibility shield as gaps in the ring of bolts will still let enemy fire through and even then, condensed fire will still overcome the Storm Shield. Consider this weapon like a Flak Jacket rather than a full suit of armor.