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Posted: 2/13/2017 2:06:18 PM EDT
[#1]
1:7, length wont matter much for stability.
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Posted: 2/13/2017 3:30:34 PM EDT
[#2]
1-7, I concur
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Posted: 2/13/2017 4:32:47 PM EDT
[#3]
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Posted: 2/13/2017 9:57:19 PM EDT
[#4]
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Posted: 2/13/2017 11:32:19 PM EDT
[#5]
Truly optimal twist depends on what projectile you are shooting, the environmental (temperature) conditions you would see, with a little nod to the barrel length as it affects velocity.
If you just want to know what's best for a general purpose AR15, the 1:8 twist is fast enough for any magazine-length projectiles (and beyond) in any reasonable temperatures and is what I would suggest. |
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Posted: 2/13/2017 11:44:24 PM EDT
[#6]
Depends on what sort of rounds you'll be shooting, and the application. What bullet weight are you planning for?
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Posted: 2/13/2017 11:50:47 PM EDT
[#7]
1:7
. |
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Posted: 2/14/2017 1:01:53 AM EDT
[#8]
Quote HistoryQuoted:
Truly optimal twist depends on what projectile you are shooting, the environmental (temperature) conditions you would see, with a little nod to the barrel length as it affects velocity. If you just want to know what's best for a general purpose AR15, the 1:8 twist is fast enough for any magazine-length projectiles (and beyond) in any reasonable temperatures and is what I would suggest. ^^this^^ What is optimum for 95 grain bullets at 1800 fps is not going to be optimal for 45 grains at 3400 fps..... |
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Posted: 2/14/2017 6:20:15 AM EDT
[#9]
It is an SBR he wont be getting 3400fps. I have 7twist in my 18" it shoots 45gr TSX just fine at 3300fps and meanwhile it shoots 90gr SMKs fine too.
Shooting shitty bullets cause more of an accuracy problem than a fast twist rate. |
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Posted: 2/14/2017 9:33:47 AM EDT
[#10]
Bear in mind that its the length of the bullet that determines optimum twist rate, ref the Greenhill formula. Longer bullets (typically heavier) need a faster twist to properly stabilize. For most purposes, 1in8 is best for most projectiles, esp those in the 52 to 55 grain range.
Also, the .223/5.56 round has a relatively low expansion ratio, so it needs a longer barrel to achieve optimum velocity, say 3000 fps or more. The 6.8 and 300BO have a much lower expansion ratio and do better in shorter barrels, 16" and less. - CW |
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Posted: 2/14/2017 9:50:57 AM EDT
[#11]
1:8 will well- stabilize any ammo that will cycle through an AR15 magazine. 1:7 was never needed except to shoot M856 tracer ammo.
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Posted: 2/14/2017 12:09:09 PM EDT
[#12]
Mmmmm tracers mmmmm good
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Posted: 2/14/2017 12:11:02 PM EDT
[#13]
Sorry to gush for a second, but damn. I love this community. The fact that I can ask a bunch of strangers for advice and get detailed information (with charts) is just freaking amazing.....thank you all.
I digress... So the build will be an SBR and if I gather what the general consensus here is correctly, length doesn't matter as much as twist rate but optimal twist rate depends on the weight of the round. Since this is not going to be a distance, accuracy driven project, and I mainly shoot 55gr non match quality ammunition it sounds like 1:9 would be optimal but 1:8 would give me more versatility to use a wider range of grain weights. Have I summed up reasonably well? |
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Posted: 2/14/2017 12:33:07 PM EDT
[#14]
yeah
get the 1:7 or 1:8 someday, you might want to shoot some 77gr 77gr HPBT is a really good defense load  Attached File |
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Posted: 2/14/2017 4:39:53 PM EDT
[#15]
Quote HistoryQuoted:
Sorry to gush for a second, but damn. I love this community. The fact that I can ask a bunch of strangers for advice and get detailed information (with charts) is just freaking amazing.....thank you all. I digress... So the build will be an SBR and if I gather what the general consensus here is correctly, length doesn't matter as much as twist rate but optimal twist rate depends on the weight of the round. Since this is not going to be a distance, accuracy driven project, and I mainly shoot 55gr non match quality ammunition it sounds like 1:9 would be optimal but 1:8 would give me more versatility to use a wider range of grain weights. Have I summed up reasonably well? Bingo. You win the reading comprehension prize.  1:9 will do you well. 1:8 might cover the bases a bit better if you do shoot anything heavier and slower.
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Posted: 2/14/2017 5:01:42 PM EDT
[#16]
Quote HistoryQuoted:
http://i.imgur.com/5F4TFAh.jpg 1/9 has no advantage. 1/7 has no liability Here's a ten round group of 52 gr SMKs I shot at 100 yards from a 1/7 BCM gov't barrel 
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Posted: 2/14/2017 5:54:27 PM EDT
[#17]
Quote HistoryQuoted:
This chart is dumb 1/9 has no advantage. 1/7 has no liability So different twist rates are not designed for optimizing different weight (length) projectiles? |
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Posted: 2/14/2017 6:12:48 PM EDT
[#18]
Quote HistoryQuoted:
This chart is dumb 1/9 has no advantage. 1/7 has no liability Here's a ten round group of 52 gr SMKs I shot at 100 yards from a 1/7 BCM gov't barrel https://www.AR15.Com/media/mediaFiles/5205/71710.JPG 1:7 does have liability. In tests by the military to demonstrate that new M855A1 ammo met accuracy claims, it could not do so in 1:7 twist barrels. That is the new 62 grain monolithic type bullet design. Switching to 1:8 twist reduced group size by 50 percent. This was also reported in a duplication of the test last year in NRA's American Rifleman. If one desires to achieve the best all around, well-rounded twist for magazine fed AR15 type 5.56 rifles, the answer remains 1:8. There is a very recent thread here with a 1/7 twist BCM short barrel, I think 10.5" that is producing 2.5-3" groups at 100 yards with a wide variety of ammo, including match grade handloads. That is more typical of BCM chrome lined barrels. Nothing wrong with that for a chrome lined fighting rifle. But it was not shooting match grade ammo any better than M193/M855 ball ammo, and most other BCM barrel owners were agreeing in that thread. |
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Posted: 2/14/2017 6:42:13 PM EDT
[#19]
post sources please
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Posted: 2/14/2017 7:46:17 PM EDT
[#20]
Quote HistoryQuoted:
1:7 does have liability. In tests by the military to demonstrate that new M855A1 ammo met accuracy claims, it could not do so in 1:7 twist barrels. That is the new 62 grain monolithic type bullet design. Switching to 1:8 twist reduced group size by 50 percent. This was also reported in a duplication of the test last year in NRA's American Rifleman. If one desires to achieve the best all around, well-rounded twist for magazine fed AR15 type 5.56 rifles, the answer remains 1:8. There is a very recent thread here with a 1/7 twist BCM short barrel, I think 10.5" that is producing 2.5-3" groups at 100 yards with a wide variety of ammo, including match grade handloads. That is more typical of BCM chrome lined barrels. Nothing wrong with that for a chrome lined fighting rifle. But it was not shooting match grade ammo any better than M193/M855 ball ammo, and most other BCM barrel owners were agreeing in that thread. Almost all of that was due to the bullet design and construction. To extrapolate that result to ALL bullets, is an unsupportable assumption. No bullet has the center of mass (CM) exactly in line with the geometric centerline, the three part M855A1, with the steel tip and core being of comparable mass, is particularly prone to having an off-axis CM. When the bullet travels down the bore it is constrained to rotate about the geometric center, so the CM rotates off axis, producing a radial force vector ('centrifugal force'). Once the bullet leaves the barrel and is no longer constrained to rotate about the geometrical centerline, it starts to rotate about the CM and the radial force vector disappears. However, during that brief time as the rotation transitions from the geometric centerline to the CM center, there is a force (the radial force) pushing the bullet radially. Now, the bullet has established a radial drift, and with no force to counter act this force, it will continue to drift away from the line of the bore axis. The magnitude to the radial force vector is a combination of the RPM and the eccentricity of the CM. Since the M855A1 was particularly bad about off-axis CM, it is not surprising that a slower twist calmed them down. However, with high quality match bullets, the eccentricity of the CM is small enough that the difference in radial 'jump' is small enough to be lost in the other factors that effect bullet accuracy. There are many groups posted on the net showing that well made barrels with 1 in 7 twist will shoot excellent groups with high quality match bullets. |
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Posted: 2/14/2017 11:52:26 PM EDT
[#21]
My first build.
I have an 18 inch barrel with a 1:8 twist. At 200 m it did a 3 to 4 inch group off sandbags with 55gr .223. At 200 m it did a 1.5 inch group with 77 gr .223. All cheap factory ammo. I was able to repeat it again. I did not think the weight would make that much of a difference. |
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Posted: 2/15/2017 12:10:04 AM EDT
[#22]
Quote HistoryQuoted:
post sources please Very widely reported. M855A1 shoots 50% tighter with slower twist |
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Posted: 2/15/2017 12:44:14 AM EDT
[#23]
Go retro, 1 in 14" & 20" barrel, just like Gene wanted it!
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Posted: 2/15/2017 1:02:40 AM EDT
[#24]
Many factors to consider. Bullet weight and length, BC, muzzle velocity, etc. As this will be a 12" barrel the same rules don't apply to an 18" barrel as the velocity will be slower and therefore will need a faster twist to stabilize. Based on my experience with pistol barrels of 10 - 14 inches in length I would opt for a 7 twist.
http://www.bergerbullets.com/twist-rate-calculator/ |
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Posted: 2/15/2017 7:52:45 AM EDT
[#25]
I think twist rates importance is way over exaggerated by many people here. Of course you are always going to have the odd barrel out there with the odd round that won't shoot well but generally it seems that a 1-7 and a 1-8 twist barrel will both shoot anything you can load into the mag fairly well. The only exception to that might be the really light 40-45 grain stuff with some 1-7 twists and even then it don't seem like I have read of many issues there.
So if you want to stay fairly safe 1-7 is good for 50-77+ but likely fine for even lighter weights 1-8 is good for 40-77 1-9 is good for 40-69 and some do well with certain 75 grain bullets 1-12 is good for 40-55 and can also do well with some 62 grain bullets as long as it isn't M855 or close to M855 length 1-14 is good for 40-55 but is pushing the envelope on the 55 The only reason I used 40 as a bottom number is I don't think anything lighter is commonly found. Even 40 isn't common although 45 is sometimes used. I could be slightly off on these but if so I don't think it is by much. While a 1-9 twist may be more optimal for a 55 grain bullet that doesn't mean it will shoot a 55 grain bullet any better then most 1-7 twist barrels. There are plenty of tests showing a 1-7 twist barrel shooting just as well or even better for the lighter M193 55 grain rounds. You are likely to have more of an issue finding what type of ammo your barrel likes then what grain to use in a 1-7 or 1-8. |
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Posted: 2/15/2017 8:51:23 AM EDT
[#26]
Quote HistoryQuote HistoryQuoted:
Quoted:
post sources please Very widely reported. M855A1 shoots 50% tighter with slower twist Here's one: When U.S. Army shooters twice fired public demonstrations of the new round, they did not employ standard 1:7-inch twist M16A2s or M4s, but accurized, match-grade, stainless-barreled rifles from the Army Marksmanship Unit (AMU). I contacted the AMU and learned that these rifles did not have standard-issue 1:7-inch barrels, but most likely (he doesn't know?) 1:8-inch twist, which probably accounts for their “match-like” accuracy. A build using accurized, match grade, stainless steel barrels (most certainly heavy profile) has much more going on than just twist rate. A squared upper receiver face, a tight barrel fit, a free floated handguard, a match chamber/throat, precision rifling, a heavy profile, and a target crown all have a greater impact on accuracy than the difference between a 1/7 and 1/8 twist- in my experience. But the author fixates on the twist rate only. I think the author is a journalist and does not fully understand what he's writing about. Secondly, M855 and the new M855A1 are junk ammo for accuracy. As has been mentioned previously, they both use a bullet made up of separate internal parts, each with different centers of rotation. Does the author intend to only shoot M855A1? Where can I buy some? what do you make of this? In 1984, the USMC conducted a test comparing the Vietnam-era, 55-grain, M193 round to the then-new 62-grain, green-tipped M855, which had boosted chamber pressure from 50,000 p.s.i. to 53,000 p.s.i. Side-by-side, 6,000 rounds each were fired through M16A1s and M16A2s with the green-tip’s hotter load. While the M16A1s saw virtually no accuracy decline, the M16A2s saw their groups more than double in size-the sole variable was chamber pressure. And the new cartridge generates even greater pressure, perhaps 3,000 to 5,000 p.s.i. So M855 was fired through the M16A1 1/12 twist barrel and saw no accuracy decline? That's total bullshit. M855 will have terrible accuracy and even keyhole in a 1/12 twist barrel. The "sole variable" between the two loads is chamber pressure? not hardly. I think his results are backwards, and therefore his conclusions are too. 
L to R: M193, M855, M856 tracer, SMK 
I'm convinced the author does not know what he's talking about, and so his conclusions are suspect. 1/9 has no real advantage, 1/7 has no real liability OP, buy the highest quality barrel you can afford. the twist between 1/7 and 1/8 won't make much practical difference (if any). I would skip 1/9 |
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Posted: 2/15/2017 9:13:57 AM EDT
[#27]
As I already said shit bullets are the problem with tight twist rates.
It works the same as a tire on your car that is out of balance, the faster the tire rotates the more extreme the effect of imbalance. An imbalanced bullet will have greater dispersion than a well balanced bullet from the same twist. So if you are going to shoot crap bullets but for some reason expect great accuracy, the slowest twist possible to stabilize that bullet is the best choice. But then if you decide to shoot heavier(longer) bullets you wont have that option because your twist isn't sufficient to stabilize the longer bullet. There are some 68gr and 77gr bullet that won't be at 100% stability from an 8twist at SBR velocities . So that is why I say go 7 twist. Here is example, mag length bullet but see the 1:8 sections, worst case, the 8 twist doesn't quite get there. I'm sure it would shoot fine but the BC may be slightly comprised.  Attached File |
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Posted: 2/15/2017 10:07:39 AM EDT
[#28]
Quote HistoryQuoted:In 1984, the USMC conducted a test comparing the Vietnam-era, 55-grain, M193 round to the then-new 62-grain, green-tipped M855, which had boosted chamber pressure from 50,000 p.s.i. to 53,000 p.s.i. Side-by-side, 6,000 rounds each were fired through M16A1s and M16A2s with the green-tip’s hotter load. While the M16A1s saw virtually no accuracy decline, the M16A2s saw their groups more than double in size-the sole variable was chamber pressure. And the new cartridge generates even greater pressure, perhaps 3,000 to 5,000 p.s.i. whoever wrote this piece is sadly misinformed. FIRST off: - In 1984, the specification for Cartridge, 5.56mm, Ball, M855, was MIL-C-63989 (AR). The specified average maximum chamber pressure was 55,000 psi measured with a piezoelectric transducer, with a three standard deviation limit of 61,000 psi. - In 1984, the specification for Cartridge, 5.56mm, Ball, M193, was MIL-C-9963F. the specified average maximum chamber pressure was 55,000 psi measured with a piezoelectric transducer, with a three standard deviation limit of 61,000 psi. In 1984, M193 and M855 had the exact same chamber pressure limits. SECOND Cartridge, 5.56mm, Ball, M193, has almost never actually been held to a 50,000 psi chamber pressure. Due the velocity requirements and limited chamber volume of the case, and inability to cherry-pick lots of propellant, ammunition manufacturers continually asked for and were issued waivers for exceeding the 50,000 psi limit through out it early production. Unofficial guidance was to allow up to 52,000 psi. By 1964 (and maybe earlier), the specification was amended to officially allow an average maximum pressure has been 52,000 psi, measure by copper crusher gauges (C.U.P.). In the mid 1970s the Army began to experiment with using transducers for chamber pressure measurement, and they found that copper crusher gauges accuracy was highly variable, and some lots of ammunition tested with copper crusher gauges may have has chamber pressure well in excess of 52,000 psi. They determined that the maximum error was around 3,000 psi. So, they amended the specification to allow a average maximum of 55,000 psi as measured by transducer because that is probably what the actually were selling all along, but if copper crusher gauges were used the measure pressure the limit remained 52,000 psi. |
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Posted: 2/15/2017 11:03:39 AM EDT
[#29]
Quote HistoryQuoted:
whoever wrote this piece is sadly misinformed. Quote HistoryQuoted:
Quoted:In 1984, the USMC conducted a test comparing the Vietnam-era, 55-grain, M193 round to the then-new 62-grain, green-tipped M855, which had boosted chamber pressure from 50,000 p.s.i. to 53,000 p.s.i. Side-by-side, 6,000 rounds each were fired through M16A1s and M16A2s with the green-tip’s hotter load. While the M16A1s saw virtually no accuracy decline, the M16A2s saw their groups more than double in size-the sole variable was chamber pressure. And the new cartridge generates even greater pressure, perhaps 3,000 to 5,000 p.s.i. whoever wrote this piece is sadly misinformed. 
PS, the author's name is John Plaster 
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Posted: 2/16/2017 9:44:34 AM EDT
[#30]
Quote HistoryQuoted:
Yes, I remember reading that article. I take issue with the author's conclusions. Here's one: When U.S. Army shooters twice fired public demonstrations of the new round, they did not employ standard 1:7-inch twist M16A2s or M4s, but accurized, match-grade, stainless-barreled rifles from the Army Marksmanship Unit (AMU). I contacted the AMU and learned that these rifles did not have standard-issue 1:7-inch barrels, but most likely (he doesn't know?) 1:8-inch twist, which probably accounts for their “match-like” accuracy. A build using accurized, match grade, stainless steel barrels (most certainly heavy profile) has much more going on than just twist rate. A squared upper receiver face, a tight barrel fit, a free floated handguard, a match chamber/throat, precision rifling, a heavy profile, and a target crown all have a greater impact on accuracy than the difference between a 1/7 and 1/8 twist- in my experience. But the author fixates on the twist rate only. I think the author is a journalist and does not fully understand what he's writing about. Secondly, M855 and the new M855A1 are junk ammo for accuracy. As has been mentioned previously, they both use a bullet made up of separate internal parts, each with different centers of rotation. Does the author intend to only shoot M855A1? Where can I buy some? what do you make of this? In 1984, the USMC conducted a test comparing the Vietnam-era, 55-grain, M193 round to the then-new 62-grain, green-tipped M855, which had boosted chamber pressure from 50,000 p.s.i. to 53,000 p.s.i. Side-by-side, 6,000 rounds each were fired through M16A1s and M16A2s with the green-tip’s hotter load. While the M16A1s saw virtually no accuracy decline, the M16A2s saw their groups more than double in size-the sole variable was chamber pressure. And the new cartridge generates even greater pressure, perhaps 3,000 to 5,000 p.s.i. So M855 was fired through the M16A1 1/12 twist barrel and saw no accuracy decline? That's total bullshit. M855 will have terrible accuracy and even keyhole in a 1/12 twist barrel. The "sole variable" between the two loads is chamber pressure? not hardly. I think his results are backwards, and therefore his conclusions are too. http://ammo-oracle.razoreye.net/AR15_com_Ammo_Oracle_Mirror_files/rounds3.jpg L to R: M193, M855, M856 tracer, SMK http://ammo-oracle.razoreye.net/AR15_com_Ammo_Oracle_Mirror_files/M193M855Impact.jpg I'm convinced the author does not know what he's talking about, and so his conclusions are suspect. 1/9 has no real advantage, 1/7 has no real liability OP, buy the highest quality barrel you can afford. the twist between 1/7 and 1/8 won't make much practical difference (if any). I would skip 1/9 NRA repeated the test, themselves, and got the same result in several different rifles. I agree that bullet uniformity plays a big role. Bullets that don't naturally balance when spinning on axis is pretty common with most inexpensive ball ammo. The problem is not limited to just the bi-metal new M855A1. That's why the slightly faster twist of 1:8 is a better choice for most users. |
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Posted: 2/17/2017 6:05:33 AM EDT
[#31]
I hope, when Molon comes back, that M855A1 is on "The List".
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Posted: 2/18/2017 12:42:51 PM EDT
[#32]
I just dug out my old copy of American Rifleman and sure enough, the by-line for the article is Maj. John L Plaster.
that's sad |
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Posted: 2/18/2017 1:17:33 PM EDT
[#33]
Quote HistoryQuoted:
I just dug out my old copy of American Rifleman and sure enough, the by-line for the article is Maj. John L Plaster.
that's sad I enjoyed his SOG book ...but 1/9 twist is the 1988 crylsler K-car of rifle barrels . |
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Posted: 2/18/2017 1:58:12 PM EDT
[#34]
Quote HistoryQuoted:
I enjoyed his SOG book ...but 1/9 twist is the 1988 crylsler K-car of rifle barrels . Quote HistoryQuoted:
Quoted:
I just dug out my old copy of American Rifleman and sure enough, the by-line for the article is Maj. John L Plaster.
that's sad I enjoyed his SOG book ...but 1/9 twist is the 1988 crylsler K-car of rifle barrels . My speculative estimation is that 99.5% of AR15s on the consumer market will never fire a bullet that wouldn't be stabilized in a 1:9 twist. |
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Posted: 2/18/2017 2:15:18 PM EDT
[#35]
Quote HistoryQuoted:
My speculative estimation is that 99.5% of AR15s on the consumer market will never fire a bullet that wouldn't be stabilized in a 1:9 twist. so what 1/7 will shoot 55gr ball just fine it also has the capability of shooting 77gr HPBT which has excellent terminal ballistics ....especially from short barreled rifles, which is what OP is asking about. maybe some of your 99.5% will come to realize it so why recommend 1/9 when it's less versatile? |
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Posted: 2/18/2017 2:18:16 PM EDT
[#36]
I don't know where you guys are getting your M855A1 but what I've shot and seen shot was exceptionally accurate from issue rifles.
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Posted: 2/18/2017 2:53:24 PM EDT
[#37]
Quote HistoryQuoted:
I don't know where you guys are getting your M855A1 but what I've shot and seen shot was exceptionally accurate from issue rifles. |
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Posted: 2/18/2017 3:33:10 PM EDT
[#38]
On a more practical note, is there even a quality production short barrel these days (sub 16" defining "short", and most likely 14.5 or shorter) that is NOT 1/7?
Nick |
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Posted: 2/18/2017 4:45:39 PM EDT
[#39]
Quote HistoryQuoted:
Define "exceptionally accurate" and "issue rifles" please Issue rifles: M16s and M4s. Exceptionally accurate: dime size groups on a GI zeroing target at 25m, fired from the prone with iron sights. For comparison M855 produces groups 2x as large under the same conditions. |
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Posted: 2/19/2017 1:30:24 AM EDT
[#40]
Quote HistoryQuoted:
Issue rifles: M16s and M4s. Exceptionally accurate: dime size groups on a GI zeroing target at 25m, fired from the prone with iron sights. For comparison M855 produces groups 2x as large under the same conditions. So about 2 MOA. |
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Posted: 2/19/2017 2:01:39 AM EDT
[#41]
Quote HistoryQuoted:
So about 2 MOA. Probably. |
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Posted: 2/20/2017 9:30:52 AM EDT
[#42]
Quote HistoryQuoted:
Sorry to gush for a second, but damn. I love this community. The fact that I can ask a bunch of strangers for advice and get detailed information (with charts) is just freaking amazing.....thank you all. You know those charts are on the internet, if you had looked there. And I bet, there are currently about a hundred million threads on arfcom asking the very same thing about barrel twist..... . |
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Posted: 2/20/2017 2:11:20 PM EDT
[#43]
Quote HistoryQuoted:
So if you are going to shoot crap bullets but for some reason expect great accuracy, the slowest twist possible to stabilize that bullet is the best choice. Back in the day in Great Britain they use to be required to shoot ball ammo in their target rifles (what we would call Palma rifles). To minimize dispersion they would use the slowest twist possible that would reliably stabilize their 144gr ball bullets. Typically that was a 1/14 or 1/13 twist for 7.62. B |
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Posted: 2/20/2017 3:39:26 PM EDT
[#44]
Quote HistoryQuoted:
Back in the day in Great Britain they use to be required to shoot ball ammo in their target rifles (what we would call Palma rifles). To minimize dispersion they would use the slowest twist possible that would reliably stabilize their 144gr ball bullets. Typically that was a 1/14 or 1/13 twist for 7.62. Quote HistoryQuoted:
Quoted:
So if you are going to shoot crap bullets but for some reason expect great accuracy, the slowest twist possible to stabilize that bullet is the best choice. Back in the day in Great Britain they use to be required to shoot ball ammo in their target rifles (what we would call Palma rifles). To minimize dispersion they would use the slowest twist possible that would reliably stabilize their 144gr ball bullets. Typically that was a 1/14 or 1/13 twist for 7.62. No bullet, whether "crap" or not, is perfect. Imperfections in the bullet cause eccentricity in the bullet flight. Imperfections in the orientation of the bullet in the barrel and as it exits the barrel cause eccentricities in the bullet flight. The faster the twist rate, the greater the eccentricities. The faster the twist rate, the longer (on average) it takes for bullets to "go to sleep" after leaving the muzzle. Rotational speed greatly in excess of that needed for stability also cause the bullet to maintain its orientation in flight instead of tracking the flight path, leading to declining BC and stability at longer ranges. Many of the concepts of accuracy and bullet trajectory are counter-intuitive to many. Here is an old thread discussing some of the intricacies of bullet flight: http://www.ar15.com/forums/t_1_5/1578254_-ARCHIVED-THREAD----Bullets-accurate-at-600yd-but-not-100yd-.html&page=1 If you are bumpfiring Wolf into a hillside none of it is going to matter. |
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Posted: 2/20/2017 3:51:31 PM EDT
[#45]
Quote HistoryQuoted:
Yes, I remember reading that article. I take issue with the author's conclusions. Here's one: When U.S. Army shooters twice fired public demonstrations of the new round, they did not employ standard 1:7-inch twist M16A2s or M4s, but accurized, match-grade, stainless-barreled rifles from the Army Marksmanship Unit (AMU). I contacted the AMU and learned that these rifles did not have standard-issue 1:7-inch barrels, but most likely (he doesn't know?) 1:8-inch twist, which probably accounts for their “match-like” accuracy. A build using accurized, match grade, stainless steel barrels (most certainly heavy profile) has much more going on than just twist rate. A squared upper receiver face, a tight barrel fit, a free floated handguard, a match chamber/throat, precision rifling, a heavy profile, and a target crown all have a greater impact on accuracy than the difference between a 1/7 and 1/8 twist- in my experience. But the author fixates on the twist rate only. I think the author is a journalist and does not fully understand what he's writing about. Secondly, M855 and the new M855A1 are junk ammo for accuracy. As has been mentioned previously, they both use a bullet made up of separate internal parts, each with different centers of rotation. Does the author intend to only shoot M855A1? Where can I buy some? what do you make of this? In 1984, the USMC conducted a test comparing the Vietnam-era, 55-grain, M193 round to the then-new 62-grain, green-tipped M855, which had boosted chamber pressure from 50,000 p.s.i. to 53,000 p.s.i. Side-by-side, 6,000 rounds each were fired through M16A1s and M16A2s with the green-tip’s hotter load. While the M16A1s saw virtually no accuracy decline, the M16A2s saw their groups more than double in size-the sole variable was chamber pressure. And the new cartridge generates even greater pressure, perhaps 3,000 to 5,000 p.s.i. So M855 was fired through the M16A1 1/12 twist barrel and saw no accuracy decline? That's total bullshit. M855 will have terrible accuracy and even keyhole in a 1/12 twist barrel. The "sole variable" between the two loads is chamber pressure? not hardly. I think his results are backwards, and therefore his conclusions are too. http://ammo-oracle.razoreye.net/AR15_com_Ammo_Oracle_Mirror_files/rounds3.jpg L to R: M193, M855, M856 tracer, SMK http://ammo-oracle.razoreye.net/AR15_com_Ammo_Oracle_Mirror_files/M193M855Impact.jpg I'm convinced the author does not know what he's talking about, and so his conclusions are suspect. 1/9 has no real advantage, 1/7 has no real liability OP, buy the highest quality barrel you can afford. the twist between 1/7 and 1/8 won't make much practical difference (if any). I would skip 1/9 Quote HistoryQuoted:
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post sources please Very widely reported. M855A1 shoots 50% tighter with slower twist Here's one: When U.S. Army shooters twice fired public demonstrations of the new round, they did not employ standard 1:7-inch twist M16A2s or M4s, but accurized, match-grade, stainless-barreled rifles from the Army Marksmanship Unit (AMU). I contacted the AMU and learned that these rifles did not have standard-issue 1:7-inch barrels, but most likely (he doesn't know?) 1:8-inch twist, which probably accounts for their “match-like” accuracy. A build using accurized, match grade, stainless steel barrels (most certainly heavy profile) has much more going on than just twist rate. A squared upper receiver face, a tight barrel fit, a free floated handguard, a match chamber/throat, precision rifling, a heavy profile, and a target crown all have a greater impact on accuracy than the difference between a 1/7 and 1/8 twist- in my experience. But the author fixates on the twist rate only. I think the author is a journalist and does not fully understand what he's writing about. Secondly, M855 and the new M855A1 are junk ammo for accuracy. As has been mentioned previously, they both use a bullet made up of separate internal parts, each with different centers of rotation. Does the author intend to only shoot M855A1? Where can I buy some? what do you make of this? In 1984, the USMC conducted a test comparing the Vietnam-era, 55-grain, M193 round to the then-new 62-grain, green-tipped M855, which had boosted chamber pressure from 50,000 p.s.i. to 53,000 p.s.i. Side-by-side, 6,000 rounds each were fired through M16A1s and M16A2s with the green-tip’s hotter load. While the M16A1s saw virtually no accuracy decline, the M16A2s saw their groups more than double in size-the sole variable was chamber pressure. And the new cartridge generates even greater pressure, perhaps 3,000 to 5,000 p.s.i. So M855 was fired through the M16A1 1/12 twist barrel and saw no accuracy decline? That's total bullshit. M855 will have terrible accuracy and even keyhole in a 1/12 twist barrel. The "sole variable" between the two loads is chamber pressure? not hardly. I think his results are backwards, and therefore his conclusions are too. http://ammo-oracle.razoreye.net/AR15_com_Ammo_Oracle_Mirror_files/rounds3.jpg L to R: M193, M855, M856 tracer, SMK http://ammo-oracle.razoreye.net/AR15_com_Ammo_Oracle_Mirror_files/M193M855Impact.jpg I'm convinced the author does not know what he's talking about, and so his conclusions are suspect. 1/9 has no real advantage, 1/7 has no real liability OP, buy the highest quality barrel you can afford. the twist between 1/7 and 1/8 won't make much practical difference (if any). I would skip 1/9 
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