I had similar problems with a 300 Win Mag Sendero Special with a muzzle brake
. First thing I did after getting the gun was to completely bed the action to eliminate that variable.
Secondly, I ordered three different weights (180, 190, 210) of Wildcat bullets thinking that at least one of them would be satisfactory. They just did not work in that gun and actually were the worst of all bullets that I tested. They would not shoot under an inch.
Next, I got some standard Nosler Accubonds to try and they were almost as poor (would not shoot under 7/8 of an inch) as the Wildcats but not quite. At this point, I started thinking about what to do about salvaging the project but could not come up with anything since I was using all the cleaning techniques, case prep, load prep that we use in benchrest shooting and still no luck. I even called the powder company to see if they had any issues with this lot # of H1000 powder and the specific lot I had was in the middle of their acceptance standards.
At this point, I was ready to turn a dream of a good deal at $600 for the gun and mounts into a not-so-good deal by having to rebarrel it. The total cost for rebarreling would have been 300 for the barrel, 200 for the barrel crown protection device and 175 for fitting the barrel. Not to mention the action work that would probably have to be done to get the gunsmith to stand by the rebarrel job.
I was sitting at the reloading desk pondering what barrel and barrel weight to use and my eye caught sight of the stack of Swift Scirroco bullets I had. Since I had nothing to lose, I decided to try some and they worked. On three successive days after finding a sweet load (standard powder charge with a specific seating depth for the ogive from the lands) and during very friendly shooting conditions, we confirmed the load by getting the gun to shoot groups in the high .1s low .2s at 100 yards (three successive days) with the factory barrel and the Scirrocco bullets. We then re-confirmed the results of the 100 yard testing during the sighting in at 400 yards.
I knew that different bullets would yield different results with match barrels and chambers, but surely did not expect the factory barrel to do that well with scirroccos since the accubonds and the expensive Wildcats shot so poorly.
Try all the different bullets you are willing to use prior to buying a barrel and sending a gunsmiths child to college. At some point, you should find one that meets your standards unless you have a bedding, stock, barrel or barrel crown issue.
Since I did the above experiment for my one and only factory rifle, I have since tuned all of my hunting partners factory guns as well. I all of my testing of their guns, it was more bullet selection and bullet seating depth than anything else. And, they are very happy with their shooting sticks. You will too once you get the magic load down pat and down range. Good luck.