When this card appeared, it was the first real big gun, with Shotgun being a little bit in a weird spot.
Unfortunately, by now it has been overpassed by a range of other weapons in every aspect:
Big damage per round: Flamethrower does 4 damage per attack, comes with more Ammo and is cheaper. It has been tabooed to
8xp6xp now though, yet if you get 1 and refill it with Ammo, you're still in a better spot.
3 damage, 3 times: Brand of Cthugha (4) is significantly cheaper, uses a vastly unused slot for , and offers the flexibility to not spend an ammo for 1hp left. It is a bit the M1918 BAR, with just a little less peak damage but also much cheaper and leaving your hands free. The only advantage of the Lightning Gun is in case you plan on refilling the ammos with Extra Ammunition or Venturer as doing so is useless on the Brand of Cthugha/M1918 BAR.
So unfortunately, no matter how cool and thematic this gun is, the +5 makes it a complete overkill that is now obsolete in the vast majority of the cases. This could still find play in Expert mode where shooting at 10+ can still be a strategy but it still requires a lot of build around to maintain a constant flow of ammo.