I install a lot of brakes on 300 RUM's and I have seen this before. What size thread did your smith use on the brake? If it's a 5/8" or smaller overtightening the brake will put a constriction in your barrel right under the threads. The verify either loosen the brake or remove it entirely and test. If the accuracy returns then you can hand lap a bit off the back of your brake and reinstall with less torque and still maintain the timing. It also may not be installed concentrically. If it's off center your groups will be all over the place.
Your barrel should have lots of life left in it. To get max milage don't shoot it hot. 3 at a time is max before you let it cool. I have inspected them with only 60 rounds through them and they had 10 times more wear than my 700 round 300Wby has. When I told him that he said he was shooting 20 shot groups as he was getting frustrated with the accuracy and just kept shooting. With his new brake he has reported sub MOA accuracy and the recoil is down to 243 sporter levels.
__________________ Build a man a fire and you heat him for a day. Set him on fire and you heat him for life.
Pretty sure it's 5/8". When I got it from the smith, the holes on top of the brake didn't quite align vertically. I got a screw driver and tightened it a little to make it "fit" better. I thought this could be a problem, so when it was grouping poorly, I took the brake off to see. It still grouped poorly. I don't think it was an issue. It's at G A Precision right now. Should get back with me this week with a diagnosis. Thanks.
Got the rifle back. GA Precision said there could be a few issues causing the erratic groups. First was the bedding might be the issue. They saw that it might have a high spot. The other was that the bolt handle was contacting the stock. The smith at the shop said he saw this in many military rifles and it caused the same problem. They corrected that issue, I held off on the new bed job since I'd rather rebarrel it than pay to bed it and not be sure. I'll dremel out the old bedding and Devcon bed it myself this weekend. Hopefully this works. If not, I'll be getting a new barrel and most likely chamber it in 300 WM. Thanks for the help everyone.
I have rem 700 in 7 rem mag that does the same thing. So I'm doing the same thing, taking it to a smith having it blueprinted, custom barrel, new stock and bedding it. Smith will guarantee it will shoot 1\2 MOA problem should be over