My father-in-law has a 257 Weatherby mag that used to shoot 1/2" or better. He did alot of load development 20 years ago to find the best, and has never changed. The gun has not been shot much, and only 2-3 times per year for the last 10 - 15 years (verify zero and shoot a deer or two). Now we cannot get it to shoot under 1 1/2 inches....The gun is bedded and barrel floated. We have tried 2 different scopes on it, but same result. I checked the crown, thoroughly cleaned it and removed any fouling, checked mounts, etc. What am I missing? Any thoughts would be greatly appreciated.
Thanks
Jasper243
This sounds just like what a friend of mine went through with his 6.5 mm Mag. He didn't shoot it much over the years and he held a tight pattern for most of that time. Then all of a sudden, accuracy fell off to almost 2" groups at a hundred yards. Since he is a stickler for details, that 2" group didn't sit well with him. He likes a tighter pattern, especially since it shot fine before..
Well......after asking around, including in the Gunsmith Forum here at GBO, we finally tracked down the problem.
Apparently, after a few hundred shots, those small bore magnums have a tendancy to erode away the inside of the barrel where the cartridge seats in the bore. Everytime you fire those things, there is a little more space and a little less steel around the projectile. Eventually the gap grows big enough that in the first milliseconds after firing the gap allows the gas to slip past the sides of the projectile, which throws off accuracy and ft/lb of energy to a degree. A Gunsmith with a bore-scope can tell you almost immediately if this is the cause. I'm told that this almost looks like a 'cottonball' of erosion, where most of the fire and pressure exists.
If this is the case, rebore to a bigger caliber, replace the barrel, or live with it. My buddy sent his rifle back to the manufacturer for a new barrel.
