An H&K L30 Gatling Laser. Designed specifically for military use, these were in the prototype stage at the beginning of the War. Multiple barrels allow longer firing before overheating. Powered by Micro Fusion Cells.
Commonly regarded as the ultimate energy weapon, the L30 saw use (post-War) by Brotherhood of Steel Paladins, UnityNightkin and even Enclave soldiers.
On that note, advanced power armors have a 90% resistance against laser damage, which means the user has to rely solely on critical hits to deal significant damage against enemies in any sort of metal or power armor. However, if the player tagged energy weapons from start on, and picked right perks, the Gatling laser can be both very devastating and economical due to popularity of ammo in later game, high per-shot damage and canceling armor AC/DR due to each shot being critical (Sniper with very high Luck).
Designed specifically for military use, the L30 was in prototype stage at the beginning of the Sino-American War. Capable of firing only in burst and a low energy capacity, it was eventually superseded by the more advanced full auto Gatling.