Early in MWO's life, I was pretty hesitant to take a mech entirely reliant on ammo-fed weapons. I didn't have a good sense of what was an appropriate ammo load, and so I relied on having some kind of energy weapon as a backup, in case I ran short. Now that the core gameplay is really well understood, we've got a good idea of how much damage constitutes a good game and how long we'll have to dump that ammo. That means we can build an ammo-dependent mech. By way of example, let's look at a couple of builds that don't work.
We'll start with a Hunchback 4G with an AC20 and six tons of ammo. (Try to ignore the crazy armor reduction.) A great game is a 1,000-damage game, and six tons of AC20 ammo only supports 840 damage, assuming you fire and hit with every single shot. Now, if you're having a really great game, landing shots and killing targets, you're going to run out early and be disappointed that you didn't have the endurance to continue.
Right, so what happens, when you add more ammo? Nine tons of ammo supports 1260 damage, which gives you room to have a great game, even without 100% accuracy. There's a caveat: Firing nine tons of AC20 ammo takes over four minutes of continuous fire (quirks notwithstanding). Solo queue matches are at most fifteen minutes long, and you are incredibly unlikely to have a solid four minutes of fire. Realistically, ammo carried that takes longer than 150 seconds of continuous fire to dump is excessive.
That sets our metrics for an ammo-dependent mech then: 1,200-plus damage worth of ammo that can be discharged in 150 seconds. That's a tall order for a Hunchback, but if you're willing to be a little unconventional, you can hit those targets.
Let's start with a build that I've actually been testing. With an XL275, you've got room for a UAC5 and an AC5 with a combined eight tons of ammo. That's 1,200 damage that can be belted out in an average of 180 seconds. As you might expect, given the metrics I mentioned above, this mech frequently finishes matches with a little ammo left in reserve and comes up short on damage, even in a good match.
Is the ammo-dependent Hunchback hopeless then? Should we just give up and go back to the 4P forever? On the one hand, yes, but we've not exhausted all of our options for the HBK-4G yet. The triple-AC2 build was always something referenced with derision in the past, but it has the potential to be a bit of a sleeper. This build can deal the same 1,200 damage as the AC5/UAC5 build and belch it all out thirty-five seconds faster. I haven't put this build through its paces yet, and the weapons necessitate repeatedly hitting single components at long range to be effective, which isn't the easiest task in the world. That said, the numbers suggest that this build is at least capable of having a good game.
So what about the mixed energy/ammo builds? That UAC20 Loki that I had previously highlighted only has enough ammo to deal 630 damage, which means that I'm leaning on the lasers to deal at least 370 damage (probably more, since I'm not 100% accurate with my UAC shots) in an amazing game. Is that realistic with six ER small lasers? All six deal a combined 30 damage within about 230 meters, so you're looking for over twelve close-range volleys in the same period where you're hoping to fire sixteen (double-tap) volleys with the autocannon. The total cycle time on Clan ER small lasers is slightly shorter than the total cycle time on a UAC20, meaning you'll probably be firing them all together and twisting between shots. Since you can do all of the necessary laser damage in the same amount of time as you'll dump your ammo, 1,000-damage games are a pretty reasonable expectation with this build.
There you go: A method to determine ammo loads, without having to lean exclusively on intuition. Your ammo load should be able to be fired in its entirety in 150 seconds or less and deal (in conjunction with your energy weapons) at least 1,200 damage in that time, missed shots and heat notwithstanding.
As a side-note on the topic of ammo, every ballistic weapon does 150 damage-per-ton of ammo, except the (U)AC10, which packs in 200-damage-per-ton. This means that when you're considering ballistic weapons, it doesn't take as much weight as you think to upgrade from an (U)AC5 to a (U)AC10, and it takes a lot more than you expect to upgrade from a (U)AC10 to a (U)AC20 or Gauss rifle, due to the difference in ammo weight.