Wheeler45, I'm not sure in a 308 but I bet it would be great. I have two of them. One is on my 338JDJ and I wouldn't be shooting it if it didn't have the MagNaBreak on it. It is like shooting a 38spl with 180grn ballistic tips. On my 338JDJ barrel, I have the break and they sent a end cap if you don't want to shoot the break and a tool to remove and install.
The other is on my XP100 in 7-08 w/15" barrel. It is loud but sure helps on the recoil. The one on my XP is on there with lock tight.
This is just my opinion but I think you would be happier with a break like the Holland or Side discharge break for the 308. I have one on my .376 Steyr and one on my 260 XP and they are VERY good breaks. Here is what I'm talking about.
Here is my 338JDJ. I'm looking for my photo of the 7-08 XP.