Machining a barrel thread adapter - Page 2
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  1. #21
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    Drill to 1/2", then use a boring bar to make the hole whatever diameter you need.

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    One of the challenges in this project is having the two machining operations on opposite sides of the adapter, concentric and in axial alignment. One the internal drill and tap, and the other the 5/8x24 threaded stub. Its easy to bore the hole between the two over sized for the bullet, but the muzzle break needs to be aligned properly to the bore. So that involves at least 3 variable, the external thread already on the barrel to the bore (concentricity of which I have no control over), then the two machining operations I perform.

    Anyone know how much it reduces the effectiveness of a muzzle brake, as the as the bore of the brake is increased over the minimum? The original flash suppressor was not machined particularly tightly to the bore diameter, and for National Match preparation, the flash suppressor is reamed oversize to prevent rain from interfering with the bullet clearance.

  4. #23
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    Here are two ways to get your internal and external threads concentric and coaxial

    1. Drill and tap one end of your adapter blank for the existing barrel threads. Firmly chuck a suitable piece of scrap in the lathe, face the end, and single-point a thread to match the end of your barrel. Screw your internally-threaded adapter blank on, and single-point your new male threads. Rough drill and finish bore the hole.

    2. More precise but a little more finicky. Chuck solid blank for your adapter, with enough stock sticking out to make the whole adapter. Rough drill through, and bore true. Counterbore to minor diam of your female threads. Csink 60 deg with a tool in the compound, not a countersink. Cut runout groove in bottom of cbore. Single-point the female thread. Support end on tailstock center. Turn end near chuck to male thread major diam, cut runout grooves at BOTH ends of the thread (the one nearer the tailstock is a starting groove). Single-point the thread. Part off in the runout groove.

    There are other ways, but these are the simplest and require the least special tooling to get good (Method 1) and perfect (Method 2) concentricity/coaxiality

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    With metric threads (and standard with slightly more math) the rule of thumb is major diameter minus the pitch. So 15mm -.75mm = 14.25mm

    Or if you want to convert the metric number to inch you can manually enter the dimensions in this website:
    UN imperial screw thread calculator

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  8. #25
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    I am afraid this project is something of a failure. The only way to make such an adapter accurately, is to machine it on the same barrel that it is going to be installed on. There appears to be too much variation between barrels to machine one adapter and have it align correctly on another barrel. I made 4 of these adapters, and the fourth one was absolutely perfectly aligned on the barrel I machined it on. When I install it on a different rifle, the range rod that was perfectly centred on the test barrel, touches the side of the brake's bore. No way to make this for universal use on metric m14's. Either the barrels have too much variation, or the threads that were made for the nut on the flash suppressor aren't machined accurately enough for aligning a brake.
    Last edited by Grizzlypeg; 07-31-2019 at 03:33 PM.

  9. #26
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    m305-brake.jpg

    Final version.qg31rnt.jpg

  10. #27
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    Quote Originally Posted by Grizzlypeg View Post
    I am afraid this project is something of a failure. The only way to make such an adapter accurately, is to machine it on the same barrel that it is going to be installed on. There appears to be too much variation between barrels to machine one adapter and have it align correctly on another barrel. I made 4 of these adapters, and the fourth one was absolutely perfectly aligned on the barrel I machined it on. When I install it on a different rifle, the range rod that was perfectly centred on the test barrel, touches the side of the brake's bore. No way to make this for universal use on metric m14's. Either the barrels have too much variation, or the threads that were made for the nut on the flash suppressor aren't machined accurately enough for aligning a brake.
    Interesting. Sounds like the barrels were just put in a 3-jaw, muzzle turned to the needed OD and then threaded.

    No bore is ever concentric to the outside of the barrel (though high end barrel makers come pretty close)- on a mass-produced military-style semi bet they're fairly crude. Muzzle threads were probably cut without a tailstock that would keep the threads concentric to the bore and not the barrel OD.

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    The good news is that it does work well, but as said, its a one off. Took the rifle to the range last night. First three shots made it clear that the scope needed some adjustment. After all that taking apart, its no wonder. Next 5 shots had me thinking the brake had dramatically improved the accuracy of the rifle. Pretty tight group (say 1.5" for 5 shots, most dispersion vertical) 15 shots later and the usual vertical stringing I get with a hot M14 barrel made it apparent that if there was any improvement in accuracy, it wasn't much. Hey, at least it didn't get worse! However, it was nice to fire a shot and be able to near immediately see the hole in the target through the scope. It clearly has reduced movement of the rifle. The nice thing is that I can always go back to the stock flash arrestor, having not modified the rifle barrel itself in any way.


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