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Posted: 10/8/2003 9:53:29 PM EDT
Will a blackhills 77 grain bullet stabilize in a 20" 1/9 twist barrel?
Link Posted: 10/8/2003 9:56:58 PM EDT
From the bushmaster website: [i]You can safely and accurately fire up to a 75 grain bullet in the Bushmaster chrome lined 1 in 9" twist barrel - even an 80 grain can be fired safely but accuracy will suffer. [/i]
Link Posted: 10/8/2003 10:39:06 PM EDT
Depends on the velocity its launched at and your particular barrel. I would say if it was NATO pressure then yes but if not its a toss up. Higher velocity, given a fixed rate of twist, spins the bullet faster, thus stabilizing it more.
Link Posted: 10/9/2003 7:44:50 AM EDT
It is not quite that simple. I don't know the exact relationship, but after fiddling with twist rates and muzzle velocities on the JBM Ballistics sight I discovered that increasing muzzle velocity will not always help stabilize a bullet. It is true that for a fixed twist rate that a higher velocity will provide a higher RPM to the bullet - which, all other things being equal, will make it more stable. Except, everthing else is not equal. When you increase muzzle velocity you increase the "ballsitic wind" on the bullet. This air resistance is the very force that works to upset the bullet (overturning moment), and is the reason that the bullet must be spun in the first place. The RPM's imparted to the bullet are a linear function of muzzle velocity. I have no idea how the wind resistance force changes as muzzle velocity increases - especially at supersonic speeds, except that I am sure that it does increase with velocity and is probably non-linear. This is all to say that sometimes driving a bullet faster COULD actually make it less stable, depending on the design and construction of the bullet. The best advice is to shoot it and see for yourself.
Link Posted: 10/9/2003 8:13:25 AM EDT
[Last Edit: 10/9/2003 8:16:46 AM EDT by 5subslr5]
ks_shooter's statement of "......shoot it and see for yourself." is good advice. If you're inquiring because you presently own a 1:9 twist barrel that barrel's sweet spot for accuracy is 69grns. If you're considering buying a new barrel and want to shoot the heavier ammunition I would consider going to a 1:8 or 1:7 twist. I checked the ArmaLite site and their Tech Note 50 gives the following for a 1:9 twist barrel: Lightest - 55grn Ideal - 68/69grn Heaviest - 75grn For a 1:8 twist: Lightest - 60grn Ideal - 75 grn Heaviest - 80 grn. 5sub
Link Posted: 10/9/2003 8:28:51 AM EDT
Only way to know for sure is to try it and see. I've shot 77gr SMK's out of my 1/9 with good success. One of the best service rifle shooters around, John Holliger, says that 77gr is no problem out of a 1/9. But it is at the upper end of the range for that twist, so trying it on your rifle is the only way to know.
Link Posted: 10/9/2003 8:34:47 AM EDT
I've shot Blackhills 77grain from my 20" 1/8 bushmaster match upper. At 100 yards I struggle to shoot MOA, at 200 yds I shoot MOA and at 300 and 400 yds I shoot MOA. I'm just trying to figure out if it's me or the ammo.
Link Posted: 10/9/2003 8:39:55 AM EDT
The 1/9 twist was originally invented to fire the M855 62 grain military ammo. It was not invented to shoot 69 grain ammo.
Link Posted: 10/9/2003 8:44:33 AM EDT
Iv'e fired both of Black Hills 77gr loads from my 20" 1:8 bushmaster with acceptable accuracy to 100 yards (not match accuraccy - but acceptable for plinking or defensive use). These were the 77gr SMK (Sierra Match Kings) loaded to SAMMI pressure; and the 77gr Noslers loaded to NATO pressure. BTW there rounds also shot acceptably from Bushmaster 1:9 twist 16" lightweight and 14.5" M4 barrels. Remember though twist is dependant on bullet LENGTH not weight. The weight figures that Bushmaster uses are guidlines - but in the end its bullet lengh that maters. BTW when I ran my test the 75gr Hornady OTM was a bit longer than the 77gr SMK IIRC - and that round also ran well enough from the 3 rifles tested above.
Link Posted: 10/9/2003 8:58:03 AM EDT
Originally Posted By Forest: Remember though twist is dependant on bullet LENGTH not weight. .........but in the end its bullet length that maters.
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Yep !! (Forrest, aren't we likely to run into length problems mostly with special purpose ammunition such as armour piercing or tracer ammunition ????)
Link Posted: 10/9/2003 9:14:49 AM EDT
Originally Posted By DevL: The 1/9 twist was originally invented to fire the M855 62 grain military ammo. It was not invented to shoot 69 grain ammo.
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I do not recall anyone saying the 1:9 twist barrels were invented for any bullet weight ?? I'm sure ArmaLite is skipping lunch waiting for you to inform them that 69grns is not the ideal weight for a 1:9 twist barrel !! (In fairness, I must have a hundred old posts saying 62grn bullets were ideal for 1:9 barrels. Finally I talked with ArmaLite management and was directed to their Tech Note 50.)
Link Posted: 10/9/2003 10:46:45 AM EDT
Originally Posted By 5subslr5: (Forrest, aren't we likely to run into length problems mostly with special purpose ammunition such as armour piercing or tracer ammunition ????)
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No so much AP (who can get M955 anyway?) but you're right on with the Tracer. Also the heavy polymer tipped bullets are more likely to give you issues, as well as any of the realy heavy match OTM bullets. The 75 & 77gr are on the hairy edge, but they do seem to work well enough. I will try some out on a cold day this winter and see how they fare when air gets denser.
Link Posted: 10/9/2003 11:03:06 AM EDT
Originally Posted By Forest:
Originally Posted By 5subslr5: (Forest, aren't we likely to run into length problems mostly with special purpose ammunition such as armour piercing or tracer ammunition ????)
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No so much AP (who can get M955 anyway?) but you're right on with the Tracer. Also the heavy polymer tipped bullets are more likely to give you issues, as well as any of the realy heavy match OTM bullets. The 75 & 77gr are on the hairy edge, but they do seem to work well enough. I will try some out on a cold day this winter and see how they fare when air gets denser.
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Got it and thanks !
Link Posted: 10/9/2003 11:03:32 AM EDT
During development of the SAW program for the US military, before we had to change the rate of twist, 1/9 was the twist selected for the then experimental M855 ammo (it was not called M855 then) after changing the tracer ammo to try to comply with an 800m burn mandated by the military the 1/9 twist would not stabilise the tracer ammo so the compromise 1/7 twist was developed. I wrote a whol explanation a long time ago and noone ever cared to remember it... [:(]
Link Posted: 10/9/2003 11:06:31 AM EDT
[Last Edit: 10/9/2003 12:08:25 PM EDT by DevL]
Here I found it. It is about to go away to the archives its so old and be cleared from the board forever... [url]http://www.ar15.com/forums/topic.html?b=3&f=16&t=145257&w=searchPop[/url] Does anyone know the differnece in overall length of the SS109 bullet and the 69 grain Sierra matchking? I bet they are fairly close with the light steel tip on the SS109 making it longer than a 62 grain all lead bullet...
Link Posted: 10/9/2003 2:22:31 PM EDT
Originally Posted By DevL: Here I found it. It is about to go away to the archives its so old and be cleared from the board forever... [url]http://www.ar15.com/forums/topic.html?b=3&f=16&t=145257&w=searchPop[/url]
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Read your previous post and everyone interested in our military should read it. Well, as you know the 6.X, 105grn round is again being considered. It's gonna stress the hell out of a platform that was never designed for this cartridge. But I'm only a dumb-ass civilian so what would I know.
Link Posted: 10/9/2003 3:01:44 PM EDT
Originally Posted By mikepenn33: I've shot Blackhills 77grain from my 20" 1/8 bushmaster match upper. At 100 yards I struggle to shoot MOA, at 200 yds I shoot MOA and at 300 and 400 yds I shoot MOA. I'm just trying to figure out if it's me or the ammo.
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Mike, you may have the same problem as me where as you shoot moa at the more distant ranges. My theory is that I can see the bullet holes at 100, so when I see the first three rounds touching, I psyc myself out and start pulling the next ones out of the group. Where as using same wpn/ammo/time/range at 200 or 300, my groups suddenly shrink to moa because I am not worrying over the group because I can't see the bullet holes at that range.
Link Posted: 10/9/2003 3:10:49 PM EDT
coldblue I think you're right. Let me change my question to, "Is a 68/69 grain more accurate coming out of a 20" 1/9 barrel than a 77grain" I shoot out to 500yds and where I shoot gets a wicked crosswind every once in a while. I'm tired of shooting curve balls with 55grain.
Link Posted: 10/9/2003 3:13:28 PM EDT
Originally Posted By DevL: Here I found it. It is about to go away to the archives its so old and be cleared from the board forever... [url]http://www.ar15.com/forums/topic.html?b=3&f=16&t=145257&w=searchPop[/url] Does anyone know the differnece in overall length of the SS109 bullet and the 69 grain Sierra matchking? I bet they are fairly close with the light steel tip on the SS109 making it longer than a 62 grain all lead bullet...
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copied this from your link: "New problem. The XM778 was now so long the 1/9 twist would not stabilize the round so the twist was increased to 1/7. The Army knew it needed to have only one type of 5.56mm ammo to be the NATO standard so when the design of the M16A2 came up it was required to fire the SAW ammo and needed a 1/7 twist to accomplish this. This is where the 1/7 twist comes from... the burn length for the SAW ammo. The XM777 and XM778 were submitted to the NATO trials, were adopted and are our current M855 and M856 ammunition." This is not the way I remember it. The XM777 & 778 were designed to stablize in the then current 1:12 M16A1's because the Army did not want to change barrels to meet the new NATO spec. These rounds were developed to complete within the NATO trails and met the minimum requirement when fired from a 1:12, but they were not longer heavier bullets like the Belgian FN SS109/L110. But it was the FN rounds (SS109/L110) & twist (1:7) that won the NATO shoot off because they greatly exceeded the minimum spec's requirement and impressed the pants off everyone. So when I came on-board the M16A2 program, I was told to emulate the new NATO bullet (62 gr.) and twist (1:7) for the new Marine Corps Service Rifle.
Link Posted: 10/9/2003 3:16:00 PM EDT
Originally Posted By mikepenn33: coldblue I think you're right. Let me change my question to, "Is a 68/69 grain more accurate coming out of a 20" 1/9 barrel than a 77grain" I shoot out to 500yds and where I shoot gets a wicked crosswind every once in a while. I'm tired of shooting curve balls with 55grain.
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I think anything with more BC is better in a cross-wind than the 55 gr. former service round.
Link Posted: 10/9/2003 3:25:10 PM EDT
coldblue you work for Knight's Armament? If so you have that 660yds indoor range?
Link Posted: 10/9/2003 8:20:18 PM EDT
I got all that info for a college paper I did. I know some of it came from a book called "Small Arms of the World" or something like that. Those rounds were developed for the 1/9 Minimi up for evaluation. Perhaps the author got the designation of the experimental rounds wrong but it was very clear the steel tipped 62 grain ammo was for use in the 1/9 barreled Minimi.
Link Posted: 10/10/2003 5:23:56 AM EDT
Originally Posted By DevL: I got all that info for a college paper I did. I know some of it came from a book called "Small Arms of the World" or something like that...
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I have found that not reference material is created equal. I have seen lots of well intentioned eronious material in print. I will check with the guy who did the Aberdeen SAW selection testing in the late 70's and see if he remembers what US round and twist was used. I think that may be what you are referencing, because by 1980, the minimi was 1:7 and the ammo was SS109/L110.
Link Posted: 10/10/2003 7:42:17 AM EDT
[Last Edit: 10/10/2003 8:35:01 AM EDT by DocGKR]
Coldblue is correct--IIRC, FN had begun working on the Minimi and the associated ammunition that became SS109 in the early 1970's. IIRC, the Minimi first debuted in Europe around 1974. At around this same time, the U.S. was working on the XM777 cartridge. A couple of years later, the NATO standardization trials resulted in the SS109 being adopted as the NATO standard, so the U.S. dropped the XM777. The SS109 had less lethality than the XM777, but better penetration of armor and helmets at longer ranges than the XM777--this was thought to be an advantage for engaging the expected Soviet threat in Europe out of the Minimi. The U.S. began testing the XM 249 around 1976-1978. Beginning in 1978-79, the M16 PIP was being developed. At some point in 1980 or so, a decision was made for standardization, that the M16A2 would use the same SS109/M855 ammo first developed for the Minimi/M249 SAW.
Link Posted: 10/10/2003 8:43:17 AM EDT
Right, Minimi development in the early 1960's, prototypes in 1974 but production didn't begin until 1982. Vriants include: Minimi Para Minimi Mk 2 M249 SAW
Link Posted: 10/10/2003 9:01:36 AM EDT
[Last Edit: 10/10/2003 9:02:36 AM EDT by DevL]
So when did the minimi have the 1/9 barrel and when did it switch over to 1/7? I would say this thread is officially hijacked![:P]
Link Posted: 10/10/2003 9:14:45 AM EDT
Originally Posted By DevL: So when did the minimi have the 1/9 barrel and when did it switch over to 1/7?
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Beats me ! The first cartridge was the M193 and later the SS109. (I have had to sink so low as to reference my copy of "Jane's Infantry Weapons (1997-98)" to find what I've posted. No mention of barrel twist.)
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