Go here and read reviews and tests. They include decibel rated sound tests on .223 platforms:
As David says, a good .308 suppressor works pretty well on a .223, when used with an adapter. It should also work well on a 6.8SPC or 6.5 Grendel.
The other thing those tests (or shooting one yourself) will tell you, is that .223 suppressors still leave a rather deafening report.
I would differ with the opinion on the gas venting. While shooting an acquaintance's AR with what I think was an AAC 7.62 SD, we got plenty of blowback straight into my eyes even with that big can. Wherever it was venting from, it was really annoying. I've also shot some ARs with a few .223 suppressors (YHM Phantom 5.56, 1 or 2 AAC models), and they were all just as annoying about venting gas back towards your face along the charging handle. SBRs tend to be worse.
Then again, take my evaluation with a grain of salt. I'm pretty pissy about gas blow back. I don't even run brakes on most of my rifles for that reason. I'd rather have the recoil.