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Posted: 11/24/2017 10:26:44 AM EDT
[#1]
Bump
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Posted: 11/24/2017 12:22:23 PM EDT
[#2]
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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Posted: 11/24/2017 4:06:20 PM EDT
[#3]
I'll try and remember to measure some barrels this weekend (all 1/7 but it should give you an idea).
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Posted: 11/25/2017 11:45:45 PM EDT
[#4]
I measured up a few barrels that are supposed to be 1/7". I got anywhere between 1/6.75" and 1/7.25" with the cleaning rod with brass brush and Mk 1 eyeball.
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Posted: 11/26/2017 2:38:24 AM EDT
[#5]
*brainfart*
I forgot about the brush. 
Thanks for helping. |
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Posted: 11/26/2017 5:19:40 AM EDT
[#6]
I'm still getting 1:9.75 with the brush.
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Posted: 11/26/2017 9:05:32 AM EDT
[#7]
I always heard +/- 0.5" too.
This is why I prefer cut-rifling whenever possible. |
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Posted: 11/26/2017 9:07:07 PM EDT
[#8]
Quote HistoryQuoted:
I always heard +/- 0.5" too. This is why I prefer cut-rifling whenever possible. |
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Posted: 11/26/2017 10:13:33 PM EDT
[#9]
Alas...my swiveling rod has a kink in the handle and now it won't swivel...
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Posted: 12/1/2017 12:14:07 AM EDT
[#10]
I measured my 1:7 at 1:7.6.
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Posted: 12/8/2017 8:30:50 PM EDT
[#11]
According to Faxon during a Q&A with Inrange, it's something like +/- 0.01"
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Posted: 12/9/2017 10:14:59 AM EDT
[#12]
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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Posted: 12/9/2017 4:02:13 PM EDT
[#13]
Regarding my technique, I'm open to more accurate ways, but we should use the same method.
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Posted: 12/18/2017 2:57:35 AM EDT
[#14]
Added two 1:8s and a 1:6.5.
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Posted: 12/18/2017 5:02:28 AM EDT
[#15]
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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Posted: 12/18/2017 5:40:46 AM EDT
[#16]
I must've been vague, or completely omitted that. Will add, thanks.
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Posted: 1/16/2018 6:18:27 AM EDT
[#17]
New year bump.
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Posted: 1/29/2018 10:52:35 PM EDT
[#18]
Added a couple newer DPMS and RRA NM measurements.
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Posted: 1/30/2018 1:53:27 AM EDT
[#19]
Quote HistoryQuoted:
How many brands of cut rifled blanks are turned into drop-in barrels? I can name Krieger and Bartlein. |
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Posted: 2/1/2018 8:55:15 AM EDT
[#20]
Quote HistoryQuoted: Rock Creek I believe is cut rifled and you can get drop-in AR barrels made with their blanks. |
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Posted: 2/1/2018 8:56:07 AM EDT
[#21]
Great thread OP, I was always skeptical of the actual twist rates. I'll try to measure some tonight.
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Posted: 2/1/2018 10:32:31 AM EDT
[#22]
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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Posted: 2/1/2018 11:14:03 AM EDT
[#23]
Quote HistoryQuoted:
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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Posted: 2/1/2018 11:48:13 AM EDT
[#24]
Quote HistoryQuoted:
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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