Is a 223 or 243 quieter when suppressed?

All things being equal, the cartridge with the smallest gas volume ( least amount of powder) and the smallest bore diameters will be quietest. Bear in mind that only subsonic rounds are truly what most would call "quiet". The rest are "quieter".

I know a guy with a YHM Titanium that when he shoots full load .308 it is hearing safe. Sounds like a .22 rifle pop.

I shoot a YHM Phantom on my 308 and it is hearing safe, haven't wore ear pro since I bought it unless I'm around other shooters that are not suppressed. The subs out of the 308 are stupid quiet and if I shot from a bolt gun would be even quieter, but I shoot from a LR308. Had a guy send me a message after this last hunters rifle challenge and ask about my suppressor, he said it sounded quieter than the others that were shooting suppressed. Love my YHM Phantom.

I also have a can that was made on a form 1 so I guess you can call it a custom suppressor. I run it on my 22LR upper and my 223 upper. The can is only 5 inches so if it had more volume it would be quieter, but I wanted something as short as possible to put on my SBR. 223 is still hearing safe sounds like a HV 22LR round, and takes away the concussion felt when shooting the 223 SBR. I've never tried sub sonic 223 rounds, I don't really see the point in them. The 22LR is very quiet when shooting with the can on.