No, AC20 Urbies are not viable. You can certainly have a good game in one and deal a lot of damage with good shot placement... but you have
every disadvantage when you run AC20 on a 30-tonner. The gun and only
one ton of ammo masses half as much as the limit for the chassis- but 7 shots of AC20 is not enough; you've
got to have at least two tons (+/-280 damage, depending on component destructs, crits, and misses) or you're effectively a dead-weight troll build. If you run with minimal armor and no jets, you can
just barely squeeze a third ton of ammo into the can, or a MLas for backup... but the sacrifices are huge.
Consider that this is the lightest 'Mech in the game that can mount IS AC20. The Locust doesn't have enough tonnage, and neither would the Commando
even if it had a ballistic hardpoint. Stuffing AC20 into a trashcan really stretches the limits of what you can do with the crits and tonnage available to you. It's a master course in shoehorning.
Here's
2 tons of ammo with XL170 and full torso armor (some shaved off legs, left arm and head stripped).
XL125 with a single jet. Having a jet makes you less vulnerable, because you can poptart with the AC20. This is what I run on my AC20 Urbie. 2 tons is still not a great amount of ammo to have, but just enough to get a few good punches in. 14 shots give you enough for a kill or three if you're careful with them. Running that light on ammo
really teaches you good fire discipline.
XL125, 1JJ, 3 tons ammo. Now we're losing torso armor. You can probably risk it as long as you have a jet so you can poptart to limit exposure time, but... the more you take off, the more you risk getting instagibbed. This config can be one-shot through its CT armor on a lucky dual Gauss crit.
Now we're back up at XL170 but with far less armor. Now we can be crippled or killed by an unlucky arty strike. Basically any serious config in the game can one-shot this.
Finally,
3 tons of ammo at XL170, minus the jet to gain FF and some more armor.
If you want backup weapons, then you need to allocate a bit of armor to your LA, which cuts into your payload space or your armor everywhere else, and IMO that is a bad decision (your mileage may vary). If you're going to run the second heaviest weapon on a 30-tonner, you should really commit to it 100%.
And now compare any of the above with an
AC20 Raven, which goes faster, still has a jet, with 24 shots and almost full armor (head and RA stripped, -1 point from RT to mask the fact that the arm's been stripped). Even
that is considered a joke build... hell, large ballistic builds on IS lights in general. The only
really viable AC20 light there is, as things stand right now, is a
UAC20 Adder, which can bring basically full armor, DHS, C-XL, a complete set of useful backup weapons,
and a proper ammo load for its main gun... only disadvantage? Low mounted gun arm.
An
AC10 Urbie can be run with
relatively little trouble, on the other hand. You just have to be careful about how you use your backup weapons because you only have SHS. The damage-per-ton ratio is higher for AC10 right now because the ammo count was raised to 20/ton... AC20 plus two tons gets you 280 damage; AC10 plus two gets you 400... and for two tons less overall, so you can run nearly full armor or bring backup weapons without making additional sacrifices. The downside is that if an enemy light catches you, then you're pretty much boned. That's kinda just the risk you take running an Urbie, though.
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