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Thread: 1:7 v. 1:9 for 55gr/75 gr

  1. #11
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    The following demonstration compares the results of firing four 10-shot groups of the same lot of 55 grain Prvi Partizan M193 ammunition from two different barrels; one barrel with a 1:9” twist, the other barrel with a 1:7” twist. The first barrel used in testing was a 16” Colt HBAR with chrome lining, a NATO chamber and a 1:9” twist. The second barrel used in testing was a 20” Colt HBAR, also with chrome lining, a NATO chamber and of course a 1:7” twist. The longer barrel with the 1:7” twist was purposely chosen for the increased muzzle velocity coupled with the 1:7” twist. Both barrels had free-float handguards.

    Accuracy testing was conducted from a distance of 100 yards following my usual protocol. The fore-ends of the weapons rested in a Sinclair Windage Benchrest and the butt-stock rode in a Protektor rear-bag. Sighting was accomplished via a Leupold VARI-X III set at 25X magnification and adjusted to be parallax-free at 100 yards. A mirage shade was attached to the objective-bell of the scope. Naturally, the wind conditions were monitored using a Wind Probe. The set-up was very similar to that pictured below.






    Four 10-shot groups of the PPU M193 were fired from the 1:9” twist barrel. Those groups were over-layed on each other using RSI Shooting Lab to form a 40-shot composite group. The mean radius of that composite group was 1.08”.

    As with the 1:9” twist, four 10-shot groups were fired from the 1:7” twist barrel. Those groups were also also over-layed on each other to form a 40-shot composite group that had a mean radius of 1.01”. The two composite groups are shown side by side for comparison. So much for the often posted nonsense about 1:7” twist barrels making 55 grain FMJ bullets “unstable.”













    Quality 55 grain bullets can shoot superbly from a 1:7" twist barrel. Most 55 grain full metal jacket bullets do not fall into the quality category, so any comparison made using 55 grain FMJ bullets with heavier open tip match bullets is nonsense.

    The 10-shot group pictured below was fired from a distance of 100 yards from a Noveske barreled AR-15 with a 1:7” twist.





    The 3-shot group pictured below was fired from the same 1:7” twist barrel for the Internet Commandos in our viewing audience.







    The next 10-shot group pictured was fired from a Krieger barreled AR-15 with a 1:7.7” twist using 55 grain BlitzKings.




    Barrels with 1:7" twists can shoot bullets as light in weight as 40 grains without issue, as long as it is a modern, quality 40 grain bullet, such as the Hornady 40 grain V-MAX.








    Here's another example of just how well a 1:7" twist barrel can shoot light weight bullets. While the group pictured below was fired from a distance of only 50 yards, it's a 10-shot group fired from a chrome lined, NATO chambered Colt barrel with a 1:7" twist using 52 grain Sierra MatchKings.







    The 10-shot group pictured below was fired from a 20" government profile Colt barrel with a NATO chamber, chrome lining and a 1:7" twist. That's about as good as a group as you will see from this type of barrel and it did it with 55 grain BlitzKings.






    A 14.5" Colt M4A1 SOCOM barrel,with its NATO chamber, chrome lining and 1:7" twist can shoot quality 55 grain bullets quite well.





    Yet another example of how well 55 grain bullets can do from a 1:7" twist barrel. This time from the 16" version of the Noveske N4 light barrel that is also chrome lined and NATO chambered.



    Last edited by Molon; 08-28-10 at 20:58.
    All that is necessary for trolls to flourish, is for good men to do nothing.

  2. #12
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    *slow clap*

  3. #13
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    My current set-up is a 16" mid length. A local shop can provide and install a Bravo Company phosphate finished 16" mid length with F-type front sight for $370. Can I expect MOA performance with 75-77 gr quality ammo with that choice? Any other reasonably priced suggestions?

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    Bravo company is pretty good.

  5. #15
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    The only problem Ive seen with light bullets and a 1:7 twist barrel was with 36gr varmint grenades.

  6. #16
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    Molon, very very well done. You have proved the wisdom my father had taught me since I was knee high to a grasshopper- Although one can make educated guesses as to performance based on the specifications of barrel & ammunition, one cannot know what the performance will be until shots are fired. Each barrel is a law unto itself.

    I applaud you, sir

  7. #17
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    Is there any benefit to spending the extra $ for a Sabre Defense barrel rather than a Bravo Company barrel?

    Judging strictly from their online graphics, SD uses the M5 designation to mean a mid-length gas system. Did I get that right?

  8. #18
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    Not all barrels are true twists. My Armalite M15A4 midlength 16" 1:9 was actually 1:8.75.

  9. #19
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    Quote Originally Posted by 298436 View Post
    Not all barrels are true twists. My Armalite M15A4 midlength 16" 1:9 was actually 1:8.75.


    That’s a very good point. I have a 1:9” twist Colt barrel that does quite well with 75 grain OTM loads out to 100 yards (though not as good as a comparable Colt barrel with a 1:7” twist.) Though I’ve never bothered to measure the twist of the 1:9” twist barrel, it is certainly possible that the twist is a little on the “tight” side of 1:9”.


    Here are some results from the 1:9” twist barrel firing the Hornady 75 grain loads from a distance of 100 yards. Before firing the TAP rounds, I obtained a 10-shot control group using Sierra 52 grain MatchKing bullets. The group measured 1.01” and had a mean radius of 0.36”. This group is a fine example of the accuracy this barrel is capable of as I have duplicated these results numerous times in the past with this barrel.

    Here is a pic of the control group.



    First up for the TAP loads was the TAP FPD load. Using the Colt 16” barrel with the 1:9” twist I obtained three 10-shot groups from 100 yards that measured:

    1.28”
    1.65”
    1.76”.

    For comparison the three 10-shot groups of the TAP FPD load fired from the Colt HBAR with a 1:7” twist barrel measured:

    1.08”
    1.15”
    1.18”.

    Using my usual procedure of overlaying three 10-shot groups on each other using RSI Shooting Lab software I obtained a 30-round composite target of the rounds fired from the 1:9” twist barrel. The mean radius for this target was 0.48”. By comparison, the mean radius of the 30-round composite target for the TAP FPD load fired from the 1:7” twist barrel was only 0.37”. This shows a definite increase in group size for the rounds fired from the 1:9" twist barrel.




    Next, following the same procedures, I obtained three 10-shot groups of the 5.56 TAP load through the 1:9” twist barrel that measured:

    1.44”
    1.90”
    2.06”.

    For comparison the three 10-shot groups obtained from the 1:7” twist barrel measured:

    1.16”
    1.38”
    1.45”.

    As before, 30-round composite targets were compiled to compare the mean radii. The mean radius from the 1:9” twist barrel measured 0.54” while the mean radius from the 1:7” twist barrel measured 0.46”. Again we see an increase in group size for the rounds fired from the 1:9” twist barrel. (Keep in mind that the control group from the 1:9” twist barrel was right on par with the control groups from the 1:7” twist barrel.)


    All that is necessary for trolls to flourish, is for good men to do nothing.

  10. #20
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    Awesome Molon!

    I always believed from "reading" that 1/9 twist stabilizing stops at about 68/69gr bullets. I find it interesting folks with 1/9 barrels drilling holes with 75/77gr bullets.

    More than a decade ago or so, 1/9s were that gap to stabilize the buku surplus 62gr bullets on the market(remember the only reason for the 1/7s in the A2s was for stabilizing the tracers)
    Glocks are functional tools and nothing else, hence they have no soul - Rob S.

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