Be sure to get a gun trust in place before acquiring a suppressor.  You don't want to have to pay a second transfer tax to move it to a trust later! 

I ended up getting a YHM 3100 for my AR. sent in the paper work back in september and still waiting...  worst then waiting for my first born to get here! haha  but i have shot through it a few times at the range there at the store. love it. 
http://yhm.net/index.php?main_page=product_info&cPath=14_18&products_id=302

My understanding is that there is an increase of a "few" decibels when using a 308 can on a 223 compared to a true 223 can.

A few decibels is no big deal in my opinion, that is why I have a gemtech qd-hvt in jail.  They cashed my check in October so I am thinking April timeframe.

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Silencerco had a video a while back testing under bore calibers in one of their Osprey.

Silencerco Osprey-
Sound Suppression               
Un-suppressed    .45 @ 163db   .40 @ 161.1db   9mm @159db
Suppressed w/    .45 Osprey   .40 Osprey   9 Osprey
45 acp        133       
40 S&W        129.7                 127.1   
9 mm        127.58                 124.9                   124.6

example:
shooting a 9mm through a .45 Osprey resulted in a measurement of 127.58 db
shooting a  9mm through a 9mm Osprey resulted in a measurement of 124.9 db
the difference is only 2.7 db -- I'm not sure if you could even hear the difference.

well the stamp finally came in and she is home. Just waiting for a chance  to test her out at the range!