Gemtec.
I would suggest the 30 cal. the reason is if you get the quick connect, you can use that same device on any caliber 30 and smaller.
With the hassle it is to get one, you don't want to have to buy more than you have to.
Leason learned and tested is that the sound is the same, or super super close when firing an AR15 with a 223 caliber can and a 30cal can. The size isn't really that big of a difference, but I would suggest the 35 db reduction models. This makes an ar sound just a little louder than a 22LR.
Getting the 30cal version will allow you to use it on your 342, 308, 223, 22-250 and so on..