Like TxLewis said, I have very little experience on suppressors (or cans) for AR15/M16 series, and here is what I know or have experiences with, and anyone has experience with other cans can chime in:
I have Tac-Ord suppressor for my M4. Since it's rifle caliber, it suppresses some amount and you still can hear the crack of the round.
Like Ponyboy said like .22LR, and there's no need for the ear plug. Some of the guys at ETH's farm shot it and I think they love it.
I think all .223 cans will vary some, as far as the noise reduction is concerned, but sometimes it is hard to tell, IMO.
I like to full-auto (FA) on my can, but I usually go in burst for 30 rds and stop and let it cool down. By this time, the can is started to heat up and the smoke/gas starts to find ways to get out ( "temperature can range between 600 degree F to 1,400 degree F after a lot of full auto fire" ) I know there are shooters going full auto for several mags or a whole C-mag, and have no problem with their can, but I don't want to risk it.
There is a great article in "Special Weapons for Military and Police" Fall 2002 discussing a newest suppressor, KAC M4QD. Basically, it states several points:
--To shot semi to maximize the service life of the can (about 5,000 rds).
--Burst only in case of emergencies.
--Full auto up to 90 rounds and let it cool off.
--Maximum, this KAC M4QD can handle 210 FA without any problem ( I guess, they mean its internal and external structure).
I personally like cans for subguns or pistols because you're really appreciated the level of noise suppressing, and the cans are lighter and/or smaller.
HTH