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1:12 twist data for a 1:7 twist rifle.

Discussion in 'Ammunition & Reloading' started by Edmon, Jun 15, 2011.

  1. Edmon

    Edmon Battle Ground Active Member

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    I have just purchased an AR with a 1:7 twist. When I look at the recipes in my reloading manuals they state they are using a 1:12 twist. Does the difference in twist effect the amount of powder I use? Are there any other differences? I would like to work up a load for my rifle.
    Thanks.
     
  2. Ben Beckerich

    Ben Beckerich NW Oregon Bronze Supporter Bronze Supporter

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    your reloading manuals must be OLD

    yea, there are differences, but i'm not a reloader, so all i can tell you is that they're there. for a non-reloading shooter, having a 1/7 twist means you'll be able to load significantly heavier bullets than the books will recommend.. and if your books are talking about a 1/12 twist, they're probably NOT talking about anythign heavier than 55gr.
     
  3. Edmon

    Edmon Battle Ground Active Member

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    I am using Bl-c2 powder and the hodgdon website states they used a 1:12 twist and the bullet weight is from 36 gr. to 90 gr. The test specs. in the Lyman 49th. edition states they used a 1:12 twist and a 1:7 twist but there is no info about any differences.
     
  4. deadshot2

    deadshot2 NW Quadrant WA State Well-Known Member

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    The data associated with a "twist rate" is for bullets in a given weight range. As a general rule, heavier bullets require a quicker twist rate than lighter bullets. Again, the "rule of thumb" is that bullets up 55 grains will stabilize and be accurate from the original twist of 1:12 that the AR came out with. As bullet weights increased the twist rates had to be increased as well. The military currently issues it's M-193 ammo to National Guard units carrying the 1:12 rate M-16 barrels. For other units the M-885 .62 gr round is standard with twist rates as quick as 1:7.

    For the average shooter, just pick a bullet weight that matches the twist rate of your firearm and load accordingly.
     
  5. sneakboxer

    sneakboxer NW OR Active Member

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    If I were fashion a guess I say, you would find higher pressure faster in a faster twist. My logic is is must take more effort to force a bullet down a tube with a faster rate of twist than a slow one. I doubt you will have any issues until you approach max if any. Work up from the starting load and check for excess pressure and you should be fine.
    Best of luck,
     
  6. nrc

    nrc Oregon Member

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    You are ok using published data for the different twists in 223.

    There is a theoretical pressure increase with steeper (faster) twists, but it is negligible.

    Nate
     
  7. Gunner3456

    Gunner3456 Salem Well-Known Member

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    FWIW, it's not the weight of the bullet that determines what twist rate it needs. It's the aspect ratio - ratio of width to length. Since all .223 are (duh) the same width, then it's only the length that matters.

    One more time. Weight is irrelevant. The reason the military went to 1:7 is because the relatively light weight but very long tracer bullets have the highest aspect ratio of all, and that quick twist is needed for them. Otherwise, the nice compromise 1:9 works well for just about all common rounds. I said common, LOL.

    I wouldn't buy a 1:7 personally because I don't ever expect to see a tracer, and it's way too fast a spin for an M193 for optimum accuracy at distance. The very fast and short bullet might tumble or it might break in flight, or both.
     
  8. Ben Beckerich

    Ben Beckerich NW Oregon Bronze Supporter Bronze Supporter

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    the only bullets that have problems with 1/7s are the super light, super hot 35-40gr fragmenting varmint loads nobody with a 1/7 would ever buy or shoot to begin with. 1/7 twist barrels will cover any and all defensive load weights, 1/9s will not. it's the 1/9s that are unnecessarily limiting.

    there's obviously different camps proclaiming different reasons for 1/7 vs. 1/9, but i've personally fired somewhere around 80,000 rounds of 55gr 5.56 from 1/7 twist barrels. furthermore, i've fired several thousand rounds of 45 and 50grs from several different 1/7s, all coming in at well above the widely acceptable standard of 2MOA for defensive loads. i've never actually witnessed a 1/7 choke on ANY load, but there's accounts of the super hot/light stuff fragmenting a meter or two from the muzzle. but i have definitely seen 1/9s choke on 69+gr. loose groups and even keyholing.

    in the spirit of fairness, i have, in fact, witnessed a 1/9 un-lined 4140 bushmaster barrel pull in .5-.6MOA groups with 77SMK handloads. i'll never claim NO 1/9 barrel can shoot heavy loads- but the odds are against you.
     
  9. Gunner3456

    Gunner3456 Salem Well-Known Member

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    When I said "common" rounds, I was thinking, and should have said, the common mil surp rounds we buy by the thousand. Mostly 55 and 62 gr. They will work better in a 1:9 twist. Also mentioning "unlined" barrels, they will almost always be more accurate than chrome lined which is needed only in fully auto weapons to combat heat.

    When you get to the heavier bullets beyond our common 55 and 62 gr, they are necessarily longer, and that length will cause them to work well in the 1:7. I believe one of the reasons the military went to 62 grain was because it is acceptable in 1:7 along with tracers.

    I would not deliberately set myself up with a 1:7 and then thousands of rounds of M193 (which I was able to get a deal on, and is also my favorite round but your mileage may vary.) That simply isn't a good 500 yard combo.
     
  10. deadshot2

    deadshot2 NW Quadrant WA State Well-Known Member

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    When discussing .223 bullets and twist rates it's important to separate AR's from Bolt Actions. One might have great success with a long heavy bullet in a bolt action because they can load the round longer but that same round won't even fit in an AR magazine. Likewise the differences in common barrel lengths. AR's from 16"-20" being common and barrel lengths in Bolt actions usually starting at 20" and going up.
     
  11. Gunner3456

    Gunner3456 Salem Well-Known Member

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    The OP opened the thread asking about reloading for different twist rates for AR's. I actually went off topic, but on the other hand, I would always consider my twist rate when choosing a recipe. I don't want a 55 or less gr bullet going through a 1:7 at 3200 fps m/l.

    I don't know of a common bolt gun made for mil spec ammo. I know only of .223's, not 5.56. I'm sure I'm about to learn, LOL. You can safely shoot .223 in a 5.56, but not the other way around.

    I also don't know of a common .223 bolt gun with a 1:7 twist.
     
  12. deadshot2

    deadshot2 NW Quadrant WA State Well-Known Member

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  13. Gunner3456

    Gunner3456 Salem Well-Known Member

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    I don't consider that a "common rifle" and I don't know anyone who owns one yet. Also note that it comes with a 1:9 twist barrel, so it isn't a common 1:7 twist gun.

    I must admit I haven't seen it before and I'd love to own one. The 5.56 chambering and the use of AR mags together with a quality bolt gun is very, very tempting. :) I'll bet you could drive nails at 500 yards with that.

    Thanks for the link.
     
  14. deadshot2

    deadshot2 NW Quadrant WA State Well-Known Member

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    It was just introduced at the NRA Convention and I predict that in short order it will become very common. Half the price of some of big names out there and a 10 round mag standard. They're supposed to start showing up this summer.

    I think I'll start re-arranging my safe and "make a hole" for one.
     
  15. Gunner3456

    Gunner3456 Salem Well-Known Member

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    You won't be alone. I love that rifle.