Yeah, its true the Gauss does more damage to individual parts, but the UAC20 has its place as well. In some situations, I'd much prefer one over the other, but this changes alot based on gameplay, opponents, map, etc.
As for the math, I stated that it was a rough example just to get a point across.
I'm curious about your Atlas example now. I'm not saying that it's wrong or right, I'm just very curious about it. I'm gonna be hammering out some math using wiki stats and stuff; bear with me.
When you say "3-times fired", is that 3 hits, or that you fired 3 times, as the latter means 6 total projectiles. Also, are you absolutely sure none of the rounds impacted the cockpit (it does have a very large hitbox, and lag may or may not have played a part)? Was there anyone else firing at the atlas/did the atlas cause heat damage to itself? Was the Atlas already damaged?
As far as I can guess in regards to armor, red means about 25% health left, which means that total damage done was anywhere from ~2625 to 3499.
Let's assume you didn't hit the cockpit, and you only landed 3 shots. That's only 750 splash damage, which is far short of the damage required, which is very curious. If one shot did impact the cockpit, that's only 1250 damage, which is closer, but still short by far. This leaves a huge gap that cannot be explained by the statistics without more information.
This next case is only if you meant "fired 3 times" and not "hit 3 times". If it was 6 shots total, that'd only be 1500 from splash; again, still short, but closer. If one shot hit the cockpit, it bumps up to 2000, which is closer to the required amount, and the extra damage could be from stray shots or self-inflicted heat damage, but it still seems a little implausible. If 2 shots impacted the cockpit, dealing 2500 damage; that makes it much more plausible. Any further than that invalidates this theory as the damage would soon exceed the total health of the cockpit, which means the pilot would be dead.
I wish I could test this further with some sort of tool, but alas time is a cruel mistress.