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Thread: Accuracy of 55gr in a 1-7 rifling?

  1. #1
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    Default Accuracy of 55gr in a 1-7 rifling?

    Just got back from the range with my older preban Model 6600 AR-15 Colt HBAR, 5.56 NATO, having a 1-7 twist and a period Colt 4X20 scope mounted. I haven't shot it for a long time and was using some 55 gr surplus ammo. Only shot at the 50 yr target and was not impressed at all with the groups and especially at that close range.
    My question is, is the 55 gr bullet a bit too short and light for that fast twist to get real good accuracy? Also should I expect the heavier NATO bullet to be noticable more accurate with that 1-7 twist? Inquiring mind would like to know.
    The rifle is in excellent condition. Ray

  2. #2
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    what brand of ammo? some of it is not all that accurate. how long is your barrel?

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    Bingo!! tump, I think you just solved the problem of accuracy. After your post I checked the ammo manufactures and found it was all mixed. Three different makers, Win, LC and one other. No wonder I got bad groups.

    Still the question remains, would the heavier bullets be more accurate then the 55 gr ones in the 1-7 twist? Ray

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    Ray,
    I would think you're working close to the edge with 1:7 at 55gr but if I remember correctly, they should still stabilize.

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    I believe at that twist rate they would be "over stabilized" and not as apt to upset as a similar round fired from a 1 in 12 twist. The Whole reason to go to the 1 in 7 twist was to stabilize the heavier and longer 5.56 bullets currently in use.
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    My ar has a 1 in 7 barrel and shoots 55gn ok, 3-4 in groups with open sights at 100. The 62 gn is slightly more accurate, both rnds are Privi Partisan.

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    Quote Originally Posted by JIMMY C View Post
    I believe at that twist rate they would be "over stabilized" and not as apt to upset as a similar round fired from a 1 in 12 twist. The Whole reason to go to the 1 in 7 twist was to stabilize the heavier and longer 5.56 bullets currently in use.
    This. You should do fine down to 55 gr., especially for training rounds. If you're scoped and going for paper punching type accuracy, then go up in weight. 1/7 can stabilize a generally wider spectrum of weights than 1/9, which is better with lighter weight bullets.

    Pat
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    you may need more practice if you haven't shot it in awhile. at 50 yards scoped, you should easily be getting 2 to 3 inch groups. that would'nt be too hard with open sights.
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    1:9 will stabilize up to 69gr.

  10. #10

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    FYI ,According to the USMC shooting 55 grain bullets in a 1/7 twist will shorten your barrel life by half .Also high copper buildup .You can do it but not recommended.

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    Quote Originally Posted by mauser202 View Post
    My ar has a 1 in 7 barrel and shoots 55gn ok, 3-4 in groups with open sights at 100. The 62 gn is slightly more accurate, both rnds are Privi Partisan.
    I have a 1:8 that gave me a 3-4 inch group at 200yards with open sights using the 55grain Privi Partisan. I am loading up some heavy rounds to play with.
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  12. #12
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    Quote Originally Posted by Deutsche Vortrekker View Post
    FYI ,According to the USMC shooting 55 grain bullets in a 1/7 twist will shorten your barrel life by half .Also high copper buildup .You can do it but not recommended.
    Is this in a Published USMC rifle manual? I can not see how SHORT, LITTLE M193 bullets will foul a barrel MORE than those LONG M855 steel core projos. There must be 4 times the bearing surface on those M855 and M856 bullets.
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  13. #13

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    Because that short little bullet is travelling at much higher velocity through a much tighter twist barrel. Anybody with the smallest amount of ballistic knowledge should understand that.Marine Corps tests showed minimum 1/7 barrel life shooting the 55 grain bullet.

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    Quote Originally Posted by ammolab View Post
    Is this in a Published USMC rifle manual? I can not see how SHORT, LITTLE M193 bullets will foul a barrel MORE than those LONG M855 steel core projos. There must be 4 times the bearing surface on those M855 and M856 bullets.
    +1 I have never read or even heard this before. Do you have some reference?

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    Be nice to know one way or another as I too have a 1/7 and face over $125.00 to get the 1/9 working... I saw on the reloading forum a place the name escapes me but it's linked in a thread about 62 grainers with 1K/$85.00 on the green tips. I have a lot of 55 grain food but I can reload if a barrel will be harmed.

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    1 in 12 was the first and 45gr was in use at the time. 1 in 10 was the m16a1 and 55gr was in use. in 1986 thay gave me the m16a2 it was 1 in 9 62gr and 64gr tracers. the army still is using 1 in 9 on the m4 and it will not develop the speed to do the job the sf is using 1 in 7 and 77gr rounds to compensate for the 800 ft per sec you loose with a 16 barrel. about 5 years ago i was talking to brian at olimpic arms and asked about shooting light bullets out of a tight twist and the answer was he has no idea why where this rumor came frome ther is no truth to it. if you are geting 3" groop's lock the rifel in a vice and run a couple of groop's that takes the shooter out of the equation and lets you prove its the ammo if you still get a loose groop after a couple of different brands then its the barrel.

  17. #17
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    The US M16 type rifles have used a 1/7 twist since the A2 version. This twist is needed for the LONG for it's weight M856/L110 Tracer projectile. The 1/9 twist will surely handle the M855 ball ammo but no USGI rifles have used this twist rate.

    Here is a good outline on the development of the M16 and it's ammuniton.

    http://firearmshistory.blogspot.com/...f-assault.html
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  18. #18
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    the colts we were issued in the 7th in and the 6in and the 8th army and 101st in were all 1 in 9 the boys in i ft lews right now are belly aching about them some thing is wrong with that write up. the first production run i personally saw was less than 6 years ago and it was not a larg run less then 600 barrels

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    Quote Originally Posted by ammolab View Post
    The US M16 type rifles have used a 1/7 twist since the A2 version. This twist is needed for the LONG for it's weight M856/L110 Tracer projectile. The 1/9 twist will surely handle the M855 ball ammo but no USGI rifles have used this twist rate.

    Here is a good outline on the development of the M16 and it's ammuniton.

    http://firearmshistory.blogspot.com/...f-assault.html
    The FN report on the SS109 was published in the UK in the late 1980s. While US gun writers were all guessing (wrongly) as to the reasoning behind the unique 1-7 USA twist, the answers they never sought out had been available for years in the European gun press. The FN report clearly states the USA 1-7 twist rate resulted from the USMC requirement for their proposed 5.56 SAW to penetrate a steel helmet at the same distance as the 7.62 SS77 projo. In fact, FN discovered the 1-7 twist with the finalized SS109 projectile could penetrate a DDR helmet at more than 1300 meters... much further than 7.62 ball could manage. Yipee!!! The USA got a 5.56 rifle & SAW that could shoot through helmets far beyond the 5.56's effective range while the rest of NATO, with their long tracers too, adopted 1-9 to 1-10 twist rates.

    FN also reported the 55 grain M193 shot well from a 1-7 twist out to 200 meters, but accuracy suffered at longer distances. Yet, the 1-7 did increase the M193's ability to penetrate steel helmets too! Amazing!!!

  20. #20
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    Quote Originally Posted by Deutsche Vortrekker View Post
    FYI ,According to the USMC shooting 55 grain bullets in a 1/7 twist will shorten your barrel life by half .Also high copper buildup .You can do it but not recommended.
    Sorry to dredge up an old thread, but has this ever been substantiated with sources. Not tryin to start any crap, just want to know because my N.M. has a 1/8 and all I've used is 55's. Luckily, only about 300 down the tube.
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  21. #21
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    Keep it dredged up I never got an answer either. My rifle seems to shoot 55s ok I haven't done any paper work but it plinks well.

  22. #22
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    My 1 in 8 twist NM AR was tearing 50 grain hollow points apart at 25 yards during an Appleseed shoot. The AMU keeps close track of shots fired in their competition rifles, and they actually count every 80 grain round shot as 2 shots due to excessive barrel wear, but they are loaded as hot as possible. There might be some truth that 55 grain bullets could erode the bore faster, actuallly not the bulllets but the hot gases generated behind it.

    And finally, the 1 in 7 twist was developed so that 62 grain tracers could be stabilized at ARCTIC conditions. At -40 bullets travel a lot slower, plus the powder is more difficult to ignite and generate less pressure. Penetrating a helmet at long range was probably just a secondary benefit.
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  23. #23
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    I have a Colt MT 1:7" 20 " A2. It shoots XM193 and IMI M193 consistantly at 2" groups or better at 100 yards. Now I reload 50 SEI sp for it for yote hunting and it will shoot them at 1" or under at 100 yards consistantly! Haven't shot anything ligher then 50 grs!
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