I worry about shatter on other guns less for perma and more for their disable capabilities. For hwacha, it would mean more rockets needed to disable x part, which would in turn nerf it as getting a full clip into someone is relatively hard.
Increasing their explosive then increases their chance to ignite a fire on their own (% chance to ignite based on damage) so that becomes an indirect buff.
I'm not saying lower any other gun's shatter damage, the Hwacha would still take the same ammount of rockets to kill components. You got me a bit confused now, I was talking about changing the damage modifier of shatter damage to the permanent hull, nothing else. So basically any ammount of shatter you have will deal 0 damage to permahull from any gun.
Ignition chances wouldn't be increased all that much by adding a little more explosive to each shot (2-3 more explosive damage), so I say that's fine, it wouldn't realy break the gun at all aside from maybe 1 extra stack of fire once in a while.
And that is to say if change is even warranted. You obviously feel strongly in favor of a nerfed mercury, but many others feel the opposite. Biased or not, I've been increasingly taking out teams who use more mercs than my side and still coming out on top. It's not an impossible thing to beat, nor overly difficult.
I feel the gun accomplishes too many roles by itself at the moment and does so with a very high efficiency for a light gun and especially a long ranged one with very high accuracy provided by a massive zoom, hence I can't help but believe the gun is not balanced as is, no matter if it is beatable or not simply because it shouldn't be able to do so many things, I don't think any other light gun can accomplish what the Merc can with such freedom, due to very restricted range, lack of zoom and straight up less damage/efficiency.
And to complete my thoughts on that last 'supposed change', I'll provide a small changelog:
-Shatter damage modifier to permahull is now 0 (from 0.1)
-Manticore Heavy Hwacha now does 27 explosive damage per shot (from 25)
-Mercury Field gun now deals 85 piercing damage (from 75) and 170 shatter damage (from 300)
Figured 170 would be better so even if the engineer can repair it with a mallet before you shoot it again, you can still destroy it with the second one, rewarding you for 2 accurate shots no matter what. Also added 2 points of explosive damage to the Manticore per shot to make up for the permahull damage loss from its shatter. Any other gun doesn't realy lose all that much.