Finally solved my grandfather's inaccurate 300 Weatherby
 

If some of you remember from my previous posts on here, my grandfather asked me for help discovering why his Mark V 300 Weatherby - the rifle he's hunted all over North America and even Africa with - had suddenly begun throwing bullets inaccurately. My dad and I borrowed it from him and tried everything we could think of, but nothing I did managed to get the gun shooting with any semblance of accuracy. Groups of 7" or more at 100 yards were the best I could do, no matter what I tried. He and I decided it probably needed a new barrel, but he wasn't all that interested in spending a lot of money fixing a gun he would never use again. His health was failing, and he had quit hunting because of it.

He died in July of 2016, leaving me several guns, including the Weatherby.

I wanted to fix the rifle, I really did, but since I'm a handgun hunter, I was hesitant putting too much time and effort into a rifle with other projects I was more interested in. However, last month my dad and I decided we needed to get it fixed. Before committing to what I thought would end up being a rebarreling project, I took the Weatherby to a gunsmith who lives nearby. He couldn't tell if the rifling was excessively worn down, but he did comment on copper buildup. He also told me the muzzlebrake wasn't a Weatherby brake - it was aftermarket.

At his direction, I disassembled the rifle, removed the muzzlebrake, plugged the barrel, and filled it with a copper solvent overnight, then thoroughly cleaned the barrel. I also confirmed that it is indeed a 1-12" twist, so the 180 grain and heavier bullets would probably not do as well. Last weekend, my dad and I took it out to test it. Using old Federal 180 grain bullets, we got it on paper at 25 yards, then 50, and then finally shot 3-shot groups at 100 yards. The 180s made 4" groups. Then, we tested two handloads I had previously made using 150 grain SSTs, one with a standard seating depth, then one seated long. The result? 1.15" and 1.09" groups, respectively. For one last test, I screwed the muzzlebrake back on tightly. The handloads' groups opened back up to 7".

While I'm somewhat put out with the muzzlebrake being such a detriment to the rifle, I'm ecstatic that I've finally solved this lingering mystery.

Wonder why it causes that to happen ?

thanks for the interesting and informative post
btw you may have fouling built up in the brake ID remove it temporarily and run an 8mm drill,
through it to be sure clearance of the bullet through it was not an issue,
then re-install it and see if it changes the results
most 30 cal muzzle brakes are 8mm internally

Interesting. Sounds like the brake is interfering with the bullet exiting the bore. Maybe it got damaged fro a drop or something.

Sometimes a brake sustains a bit of damage to one or more of the exit holes. This can actually cause enough backwash, turbulence, whatever you want to call it, within the brake to destabilize a bullet just a tiny bit, but enough to cause great inconsistencies in impact points. I've seen a few BOSS systems go through this nightmare. It's virtually the same thing as a slightly damaged crown and causes almost identical results.

I'm probably not going to bother with it. Maybe just get a thread protector. I've got a Lead Sled, so the recoil won't bother me, and I wouldn't really feel it in a hunting situation anyway.

this can be a food for thought , but I know when a GOOD smith installs a brake, they center the bore to the barrel contour and NOT get tread the end of the barrel, as not all barrels are TRUE to the center of the bore
SO< maybe if a smith took a shortcut or?? it could be off a hair and causing the issue's here!

Lone Wolf, please do not subject that beautiful stock to a lead sled. I have seen several stocks damaged because of them. A stock is designed with a human shoulder movement to absorb impact in mind. A lead sled allows way too much pressure and causes a wedge effect between the barreled action and stock. The old man built a few stocks for people with big boomer magnums that got split on a lead sled and he told them in no uncertain terms not to subject his stocks to that crap. If you can't handle the recoil of whatever you are shooting then either learn proper form, get a proper fitting stock, or don't shoot that particular firearm. I honestly have a hard time believing lead sled is still in business with all the reports of split stocks I have seen for myself and online.

The thing about muzzle brakes, much like T/C QLA, is if its not centered it can cause accuracy issues.
You hear pros and cons about barrel break-in. And in your case, this is a strong case for it and my reasoning for it. My thoughts is that a break in is necessary for better accuracy. Not right out of the box but down the road. Had a break in been done initially, the minor burrs and scratches in that barrel would have been removed allowing for easier cleaning with less copper build up after shooting. As you proved it was this build up plus the muzzle brake that caused the accuracy issues.
You may want to have a smith either remove the threaded area of the muzzle and re-crown it or install a nut over the threads to give it a finished look. Glad you got it shooting again.

I can attest that this is something to be aware of. I found out the hard way and damaged the butt plate on a rifle. Luckily not the stock. I now use a lot of padding around the stock when using it.