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Posted: 11/23/2017 3:33:42 AM EDT
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According to Douglas, a premium button rifled barrel maker, "NOTE: Our twists are at the whole inch (i.e. 9,10,11, etc.) and tolerance is plus or minus 1/2"." My technique:
- Use a straight edge and mark a witness line along the length of the rod. Mark the top-middle of the rod guide. (black or silver Sharpie) - Tighten a worn, but snug, ~1.5" long .22cal bronze bore brush into a one-piece rod. - Start the brush with the line on the rod facing straight down. Insert until the brush begins to rotate and the line comes up to top dead center. - Make a horizontal mark on the rod just behind the guide. - Push rod until the witness lines are together again. - Make another horizontal mark on the rod just behind the guide. - Remove rod and measure between the two horizontal marks. This is the distance for one revolution, I.E. 9:1. If the brush loosens, or the rod binds in it's rotation, or something just doesn't feel right, restart. Current Arfcommer measurements: 1:6.5" - CLE NM 6.45* 1:7" - 7.25 - Core15 7.60 1:8" - Lilja Recon 7.70 - DPMS NM 8.19 - RRA NM 8.43 - DPMS NM 7.81 - RRA NM 8.06 1:9" - S&W 9.75 - CMMG 9.75 |
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I’ve got a 1:9 twist 20” Colt H-Bar, never measured the twist rate but it won’t shoot 75 or 77 grain bullets accuurately enough for me. My 20” Colt H-Bar shoots hand loaded 69 grain SMK very accurately. I have only seen one 1:9 twist barrel shoot 75 grain A-Max accurately but that was at only about 50 yards. I suspect the accuracy of the 75 grain A-Max would deteriorate by 200 yards. I’ll be surprised if you achieve decent accuracy with 75-77 grain bullets unless you shoot at high elevation. I normally shoot around 350’ ASL and I leave my 75-77 grain bullets for my 1:7 twist barrels. Can’t hurt to try and please post your results. I do agree there are manufacturing tolerances and not all 1:9 twist barrels are exactly 1:9. I just have never been around on that would shoot the heavier 75-77 grain bullets. |
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Good observations, OP.
Just checked two barrels out of curiosity. A CMMG 1:9 measured slower at 1:9.75. Helps explain why it will not shoot 77SMK well, and other 1:9 barrels do. My Lilja match grade Recce 1:8 measured 1:7.70. That seems optimal to me. Always good to know actual twist. |
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I would start the brush with the line on the rod facing straight down. Insert until the brush begins to rotate and the line comes up to top dead center, then mark the rod. Continue to push the brush until you get 1 full revolution, mark the rod, remove and measure.
This should ensure that even with a 1:7 twist barrel that you insert at least 3.5" into the rifling before you start to measure, ensuring the brush has full engagement and does not skid. |
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Quoted:
It's a very neat idea to check, but I'd be just as suspicious of tolerance in the home-testing method than in the factory work. |
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Quoted:
Probably not precise to the hundreth, but accurate enough to determine if the twist is rather bang on, or slow or fast. Just another tool in load testing. Now sure, if the gun is marked 1/7 and the home-brew measurement comes in at a consistent 1/10, THAT would be good data for sure and certainly worth a call to the manufacturer!! |
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