You mention that it's "only" 1 bullet less with SP, that's how it is with every weapon. You're assuming that with the ACR that you're 100% accurate though, which isn't true. Look at your accuracy rating. While it isn't exactly accurate (I shoot alot when I know I'm not hitting a person) even saying that you're 20% accurate that means that shooting at someone in a firefight you're only hitting them with 1 out of every 5 bullets. We'll even say 1 out of every 4 for you more accurate people. That means without stopping power at a decent range on a moving target you've got to shoot at least 20 shots at them to kill them...(and honestly you're probably going to shoot alot more than that). With stopping power it would be 16. It sounds like not much of a difference but trust me, on maps like Derailed when someone is running near their spawn having that 1 less bullet to kill is often the difference between +100 and nothing.
But that accuracy stat is measured across all guns in all situations. It's ridiculous to imply that because your overall accuracy is 20%, you'll have to fire 16-20 shots from an ACR in order to kill someone, every time.
The point is that the ACR's near-zero recoil and high rate of fire means you'll be killing people very quickly and easily with it. Compare a plain ACR to a plain SCAR (let's say both without stopping power). The ACR takes 4-5 shots to kill vs. the SCAR taking 3-4 shots, but the ACR may well place more hits on the target in a shorter time frame because the gun isn't bouncing around from the recoil and it's putting out roughly 4 bullets in the time it takes the SCAR to fire 3. Now, that's only assuming you are perfectly aimed. Some people may prefer a gun with a little more recoil so at least some of the shots hit without having to aim perfectly, while others might rather have a gun with near-perfect accuracy like the ACR so they can adjust their aim while firing without recoil to throw them off. It's just a trade off that depends on how you play.
Unfortunately there's no detailed online stat-tracking like there was with World at War. You could get detailed stats for each and every weapon used and figure out which ones you do best with and which ones you're not good at. You just have to get a feel for what you like and go with what you think you're best at.