Hahaha this is so true. My first can was a 762SD, and I've run it on 308,6.8,223 and 300 BO. But I wanted a smaller dedicated can for my 223 rigs, and then picked up a M4 2000...
I have a Specwar 7.62 CAN that I run on my .300BLK, .308 gas gun, and a 5.56 AR. Personally, even though the 7.62 CAN on a 5.56 may technically be a few decibles louder, to me it just sounds different, and not necessarily louder. I actually prefer the sound of a 7.62 CAN on a 5.56, over a 5.56 dedicated CAN. But probably just personal preference.