I mean current main battle tanks have guns that supposedly do an equivalent damage to an AC10. Yet a quick Wikipedia of the M1 Abrams for example reveals they can effectively make kills at over 2.5km.
So just look at those numbers. A current main battle tank has a main gun roughly equivalent to an AC10 but can make 'kills' at 6.25 times the ac10's effective base range. Granted armour levels would be different but still.
So the obvious answer is that Battletech and mechwarrior weapons are considerably more powerful than current weapons so much that the AC10/120mm gun comparison simply isn't true with such huge calibres (or masses in the form of missiles) that the projectiles can't go that far. Weapon weights without ammo would lend some credence to this. If that's the case a Heavy machine gun should be capable of killing a current main battle tank surely. MG's another weapon that has laughably short range.
Current missiles like the hellfire, again Wikipedia are capable of just reaching five miles (8K) away and considering they can be mounted to drones shouldn't be massively larger than the small LRM's that mechs use albeit in a cluster.
Energy weapons though......well that's another matter as i don't know much to compare it too but again seem pretty poor in range.
What i'm getting at is if a mech appeared in current day times in the distance i'd imagine a couple of Abrams/challengers/leopards or even an Apache would of killed the thing or at least messed it up before it even got into effective range! Gauss and ERPPC's would be the mechs only chance at actually doing noticeable damage.
Edited by mad kat, 13 April 2018 - 10:50 AM.