2
Why it's trash is fairly simple:
GR: 15 damage for 15 tons (1dmg:1t), 150 potential damage per ton of ammo.
HGR: 25 damage for 18 tons (1dmg:0.72t), 125 potential damage per ton of ammo.
LGR: 8 damage for 12 tons (1dmg:1.5t), 160 potential damage per ton of ammo.
For 12t you could run an AC/10 or two ERLLas + 2 DHS. The range and cooldown are nice, but nice doesn't make up for being atrociously inefficient. It's really not helped by the fact that anything that can carry dual LGR can carry a much more effective payload.
Meanwhile, on the other end of things, anything light enough for a single LGR to be a potential consideration isn't going to be able to hump enough ammo to do much damage, especially when you have to consider the likelihood of actually firing off and hitting enemies with 100+ rounds. Doubly so when you could just run HPPC/2x PPC/laservom/a UAC/5, &c. for better damage with less effort.
To be worth taking LGR would need to be altered to fit the same curve as the HGR, with ~175 potential damage per ton of ammo. Whether that's due to decreased tonnage, increased damage, or increased ammo per ton would affect what it would be useful on. Bumping to 22rd ammo bins wouldn't help much, but it would be something.
Honestly though, LGR suffers from AC/2 syndrome: it's too weak to be a useful weapon on larger 'mechs and too heavy to be a primary weapon for lighter 'mechs.
Edited by Alexander of Macedon, 18 May 2018 - 11:34 AM.
