From Mechwarrior Living Legends Wiki
Ballistic Weapons are weapons that fire projectiles, as opposed to Energy or Missiles, and function as the basic "guns" in-game. They have modern-day equivalents, like cannons, shotguns, sniper rifles, and machine guns, making it easier to grasp how they're supposed to be used in-game.
Ballistic weapons differ in their usage depending on their type. Small auto cannons excel at anti-aircraft roles and are good at destroying Battle Armor and damaging Light Mechs. Their high rate of fire along with their extreme range and ammo capacity make it easy to hit these evasive targets for consistent damage. Higher caliber rounds have much shorter range and longer reload times, making them more suited for destroying slow and heavily armored targets. They are still effective at destroying light mechs and aircraft, but at a much shorter range.
Gauss rifles are almost exclusively used for destroying ground based armor. They have a slow firing rate and limited ammo, making them less suited for anti-aircraft purposes (though a hit against one will do significant damage). Gauss rounds have a fast, but not instantaneous travel time, which makes them ill-suited for engaging battle armor. Like the auto cannons, the lighter rounds have longer range and the heavy rounds have shorter range.
Ballistic weapons for the most part have low heat output and long range, though those attributes vary depending on the size and type of the round. The largest rounds in particular have quite short range made for brawling. Many ballistic weapons are well-suited in an anti-aircraft role, whereas missiles and energy weapons do poorly for various reasons.
Because ballistic weapons fire a projectile, there is a learning curve in establishing the correct lead on a target in order to register a hit. Ballistic rounds also consume ammo at a fast rate, which means that mechs either need free tons or need to return to base in order to continuously fire their weapons. Some ballistic rounds can also overheat the weapon itself (such as the auto cannons), which involves an aspect of micro-management that isn't found on energy weapons or missiles.