Most 'Mechs do not actually mount LRM20s. A lot of them mount 20 missiles, but not all in one package. Granted some of them do, like the Mad Dog, but not always. Even dedicated missile boats like the Catapult do not always mount LRM20s.
Even better, the Fafnir mounts two HEAVY Gauss rifles

. Although, once again, the others that mount them rarely mount another very large gun.
The
Adder (it is not the Puma

) mounts two in it's Prime configuration only. Even better, they are ERPPCs. Anyway, let's not forget the Hollander, which is like an Adder, but it has a Guass rifle

.
Now that that is over, let me ask one question. Why are we comparing these? Has this come down to a "My universe is better than your universe?"
Anyway, for perspective on the pain these babies dish out: The main gun on an M1A2 Abrams is considered a medium rifle in BattleTech. It does a grand total of three damage to BattleTech armor. For comparison, a PPC (non-upgraded) inflicts ten, and a gauss rifle 15. A
heavy gauss does something on the order of 25 at close range. Put in perspecitve (again) a ton of armor protects against 16 damage for standard armor. FF is slightly better, at 18 for Inner Sphere and 20 for Clan.