It's very simple. The PPCs are quite high-mounted in the torsos, and the convergence with the AC20 is very decent. After quirks and a few skills, the cooldown on your 40-damage pinpoint volley is about 3.25 seconds, and the heat efficiency is very manageable. Obviously the armor quirks on this thing are BEASTLY so use those shield arms liberally. After skills (I get 8 out of 10 Armor Hardening), my armor values in the arms and legs are 41 and 71 respectively, with the torsos maxed of course, and the head at 19. If you want to take 10 points out of each leg to put in the arms, it'd still be fine I think. It's also worth noting that I have both Magazine nodes for this thing, so with 4 tons it's 36 rounds of AC20. This is lots in my mind. At 40 damage per alpha, 36 alphas is 1440 damage (with optimal range), and even if and when you run out, the SNPPCs do reliable damage with no minimum range. One could obviously move the PPC out of the right torso into the arm, and fit a LightEngine for more speed and an extra DHS. But I personally think losing that high mount isn't worth it. Of course, it's still not much lower than the AC20 (a bit, but not much) so it's worth considering; though that also means you'd need to rearrange some more armor, probably out of the legs, to keep that right arm alive (you also get an extra half-ton of armor from the engine swap, so that factors in too).