Let me put it as bluntly as possible: according to TT crunch and fluff, none of the ACs in MWO are UACs; the IS UAC/5 is a RAC, and the cUACs are amalgamations of three different types of AC. They have one RAC for the IS, LB-Xes, and a shitload of AC variants.
There's a whole lot going on re: targeting computer use, tonnage/crit slots, ammo per ton, increasing miss chances, &c., but the fundamentals are this in TT: AC = 1 shot per turn. UAC = 2 shots per turn. RAC = 6 shots per turn. When a UAC jams you cannot clear it in the field. When a RAC jams you can take a turn to try to clear it.
Ofc. there are also differences between different editions, but that's the fundamental.
The whole thing going on with c"UAC"s? In TT those are
all ACs. Because that's how things are in TT, some ACs deliver their damage in single shells, others deliver it in bursts of smaller shells. They took the latter model, gave it the ability to fire twice per "turn" like UACs, and then slapped the RAC jamming mechanic onto it. I mean, yes, technically you could argue that the double-burst is actually just a UAC with the RAC jamming, but... why? They're already drastically simplifying things and treating all ACs and cACs as being single-shell variants, why change the cUACs to a different firing model?
Game balance. That's pretty much the only reasonable interpretation I can give for why they put in all that extra effort when cUACs could have behaved exactly like the IS "UAC"/5. They didn't push the timeline far enough to give the IS other sizes of UAC, so they needed to not make the Clan ones totally overpowering (but also didn't want the vidya-unfun mechanic of having a misfire mission-kill your main armament). The problem is, the cACs are
still worse than the cUACs, markedly so, even in areas where they should be advantaged, like tonnage/crit slots. PGI have already shown that they're willing to cut, snip, and paste over established mechanics for game balance, so I really don't get why they haven't given the cACs a balance pass; it's bad enough that I don't think I've actually
ever seen a Clan 'mech carrying cACs. :S
Like, they jumped through all these convoluted hoops to make cUACs work in the context of MWO, and yet they've put basically zero effort into cACs apparently. You don't even
see Clan players running ballistic builds that are anything other than Gaussboats, stuffing an Assault full of LB-Xes, or stuffing as many cUACs as possible into their 'mech of choice. Meanwhile IS 'mechs, despite being markedly more limited in their choice, run pretty much every ballistic weapon from AC/2s on up and can do so without avoiding flat-out better builds for the chassis they're one. Some even *cough*Black Widow*cough* deliberately and effectively mix UAC/5s with standard ACs!
I'd love to see cACs be worth using, because I like running my Mediums with ballistics for that unexpected punch, but frankly most of them don't have the tonnage/slots for ammo to make cUAC fire sustainable, cACs are too big (leading to the same problem), and Clan energyboating is so rewarding that there's not much reason to run dakka unless you've got a Kodiak or Direwhale to run four fat ACs in.
Like, take this: You can run a CDA-3M
comfortably with a UAC/5, three tons of ammo (90 shots!), ECM, two MLs, and an XL-280 with an extra heatsink inside, stripping only six points of armor. What Clan mediums can you run a cUAC/5 on? The Vipers are 40 tonners as well, but they're stuck with a bunch of hardmounted shit and can barely carry the gun and 1t of ammo. The Ice Ferret is a 45 tonner, but it can barely squeeze in a second ton of ammo. A Shadowcat can manage the exact same build but carry enough ammo for 10 shots more, that's pretty good... but it's a 45 tonner and has to carry the AC on one of those easily-shot arms. It runs a solid 15kph slower, too, though MASC and JJs make up for that. A Stormcrow could do the same, except with worse mobility and more spare space. That's two contenders. A Nova or Hunchback IIC could do it easily, but the latter isn't an omnimech and both are 50 tonners.
So, uh. A mediocre IS 40 tonner can run a build that the Clans need a strong 45-50 tonner to manage, which could mostly be doing better builds. Meanwhile on the IS side I could fit a fucking
double UAC/5 build into a
Blackjack. I could run a UAC/5 with 90 rounds and 2x ML backup on a fucking
Raven and still do 127kph. I could do the same with 75 rounds on a
Commando, for crying out loud. In Clan 'mechs you can do it on the Adder, but that has functionally the same situation as the Stormcrow -> CDA where you're slower and the gun is hanging out on an arm waiting to be shot off. You could do it with a Mist Lynx, but that leaves you in the unenviable situation of playing a Mist Lynx. Which is also pretty much the opposite side of the Cicada's coin - the Cicada is a strong light pretending to be a mediocre medium; the Mist Lynx is a shit medium pretending to be a shit light.
And remember, the UAC/5 is
more tonnage/slot-intensive than the AC/5. The opposite is true for cUACs and their cAC equivalents. If you use IS ACs instead of UAC/5s you can fit even more on.
So basically if you want to run dakka on a Clan light/medium your options are limited to 45t+s, with maybe three builds where you get to place facetime with easily blown-off arm-mounted guns. The SHC would be okay, I guess... but why not just run cGauss, cERPPCs, or cERLLs? Same with Stormcrows and laser vomit. Any of those would out-damage, and you don't have the speed or quirks to really abuse the dakka like IS lights/mediums often can. Not to mention that you're practically forced to use cUACs if you do because the cACs are generally too big and you don't have the quirks to make them work.
That last bit is all just personal ranting about wanting to run small dakka on Clan 'mechs and not really being able to effectively, ignore it. Fuckin' balance problems everywhere that make half the potential builds in the game trollish or impossible.