Jim Harmer bought me his problem child Mossbrg Patriot rifle. I showed a full transformation, but at the request of my viewers, returned it to its original form and went about accurizing it without swapping new equipment.
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About the Rifle
Jim Harmer’s .308 Mossberg Patriot has given him issues for years. Even though it was returned to the factory, it still doesn’t shoot well. Something just isn’t right with it. I took some time to evaluate it. My best 100-yard group measured 3.8”, confirming Jim’s findings.
I completely rebuilt the rifle in a different caliber and achieved the results I was looking for, but I still hadn’t solved the original problem. (If you’re interested in learning hands-on gunsmithing, consider attending the Colorado School of Trades!)
I returned it back to its original ill-shooting form and got to work!
Troubleshooting
The first thing I did was secure the barrel in the lathe and borescope it with a Lyman Borecam 2.0. My footage didn’t reveal anything extraordinary, just a run-of-the-mill budget rifle barrel, but it definitely wasn’t the worst I have seen.
I indicated the lands and the grooves ahead of the throat area and got them running true. I also measured chamber runout— the alignment between the chamber and the lands and grooves. It was between 0.001” and 0.002” total indicator reading, about what I’d expect for a factory rifle. This is much more than the 0.0001” I target while building custom rifles.
My biggest concern was still the way the stock was constructed. The plastic “bottom metal” has corrugation on the bottom that sits directly on top of the stock, sandwiched under the action in the front. Two plastic petals, the action screws, and the front end of the trigger guard all directly compress in the rear. As things compress, the plastic petals will bend and spread wider. None of this is conducive to consistency and thereby accuracy. Failure to strip rounds off the magazine is another fault of this gun. The bolt would drag across the top of the cartridge 30% to 50% off the time in both Jim’s and my experience.
Bedding
No surprise, my first order of business was to fix the stock situation.
I cut the extension of the plastic bottom metal, filed the edges, and created an epoxy bed inside the stock where it used to sit.
I also drilled into the trigger guard to craft front and rear shouldered pillars.
These changes made for a stress-free, stable bedding platform for the receiver.
I reinstalled the .308 barrel and checked headspace before bedding the barreled action.
When all this was complete, I confirmed I could adjust the torque on the front action screw and not feel appreciable movement between the barrel and forend. I immediately noticed a huge improvement.
I thought for certain this would lead to improved accuracy — I couldn’t have been more wrong.
Not all of the shots even hit paper.
Though the upgrades are certainly an improvement, bedding didn’t change the range results indicating it wasn’t the root problem.
Second Attempt
Searching for more answers, I removed the factory barreled action from the stock and installed it into the MDT LSS chassis I had used for the transformation.
This leaves the factory .308 barrel as the only difference from the rebuilt 6mm Dasher I had great results with.
The .308 still displayed the same awful accuracy in its new configuration, narrowing the root problem down to the barrel.
I had previously inspected the crown with Lyman’s Borecam 2.0 and didn’t find anything amiss. The chambering job appeared “decent for a factory rifle,” so I believe it is the bore itself.
I suspect there is considerable dimensional variation as the bullet travels down the bore. I also found a slight enlargement using a pin gauge at the very end of the muzzle. This isn’t unusual with button-rifled barrels.
Conclusion
I could lap the barrel in an attempt to improve it, but given the consistent six-inch “groups,” I don’t believe it is currently worth the effort.
Though not everyone agreed, it turned out my first inclination to scrap the barrel completely was right. This project has been time-consuming but also goes to show that the bullet itself and the bore are the two most important factors in accuracy, something I’ve long believed.
In one of my recent conversations with Bryan Litz, I also learned that the ratio of the energy of the projectile to the overall weight of the rifle has a significant influence on group size. A heavier rifle with a low-power cartridge (ex. .223) will be easier to make accurate than a lightweight .300 WIN Mag. Every barrel will have its own personality, and shoot some bullets better than others. Take the time to experiment and don’t underestimate the importance of your barrel!
If you’re interested in learning more about gunsmithing, consider attending the Colorado School of Trades. If not, consult a qualified gunsmith!
Get the Gear
Subscribe to @backfire on YouTube and backfire.tv to keep up with Jim’s adventures!
Jim has also developed a cool recoil-reducing rifle buttpad called the Backstop. Check it out HERE.
Precision Matthews PM-1440HVT-2 Lathe
Element Optics Theos 6-36×56 FFP
Longshot LR-3 2 Mile UHD – $899 MSRP
Lyman Borecam 2.0: Wireless Bore Camera
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Thanks,
Gavin Gear
