Why a 1/9 twist barrel?

[Walkalong]

An AR15 in .223/5.56 with a 9 twist is ideal for most of the bullets loaded in factory ammo..

This. Unless you want to shoot the heavies, say 70 Gr and up, 1 in 9 is good to go.

 

[Sunray]

Military fiddled for years with the rifling twist and bullet weight until they found a combination that was giving 'em close to but not exactly the results of a .30 calibre in the 5.56. Every time they changed bullet weights and rifling twists civilians jumped onto the band wagon. Now, there are people who want to hunt deer sized game with a .223 AR. So the end result is hordes of rifling twists and bullet weights. The latter isn't exactly a bad thing. The former is more about what you're doing with the rifle.
Mind you, a ground hog won't care if it's killed with a deer bullet. A deer needs the right bullet for the hunter to be ethical though. Most light .223 bullets are varmint bullets that are designed to expand rapidly upon impact with little penetration. Deer bullet penetrate as they expand. Plus you get all the stability issues that apply mostly to longer ranges than deer hunting.

 

[JeeperCreeper]

The quick and dirty way that I was told and read over the years... 1:9 for 55 gr and 1:7 for 62 gr. Obviously that is over simplified, but it makes it easy to remember the concept.

All my AR's are 1:7 because clearance sales... they do seem to favor 62 gr. But I have also gotten great groups with 55 gr. I'm talking budget ammo here, not match grade. If I were just plinking, I'd still get a 1:8 or 1:9.

 

[MistWolf]

https://www.thehighroad.org/index.php?threads/are-my-bullets-disintegrating-in-flight.821778/ this is what I found on the subject without looking, and while 52gr is not 55gr, its fairly close, and he's also producing velocity well below the max for a 55. I had numerous 55's explode out of a 1 in 7 using commercial bullets. 1 in 9 did fine. I think military bullets would be fine, but military 55gr bullets are not exactly the standard.

Completely different bullet construction. 55 gr FMJs are the most common bullet used in ARs

 

[Walkalong]

The quick and dirty way that I was told and read over the years...1:9 for 55 gr and 1:7 for 62 gr.

The only 62 Gr bullet the 1:9 may have a problem with is the tracer, because it is bullet length, not weight, when it comes to what twist it needs.

The 1:7 AR barrel is a relatively new thing when it comes to ARs.

 

[boom boom]

This guy has an even better calculator for barrel twist and bullet stability than in my earlier link. He also has a database of most reloading bullets online regarding bullet length. Used it to check out potential 6.8 SPC loads.

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

 

[Damon555]

The only 62 Gr bullet the 1:9 may have a problem with is the tracer

Which leads to the question.....Other than the military folks on here how many of you shoot tracers out of your AR15? For 99% of AR15 shooters a 1-9 twist will be all you will ever need.

 

[FL-NC]

may do fine out of a 14', may do fine with heavy jackets, but I have personally seen 55's explode into dust at the 75 yard mark with enough consistantcy to demonstrate for other people, and its not exactly an unheard of thing. You can find THR posts about it. I was chronoing 55 grain bullets at about 3300 FPS at the time and was keeping them in 2' at 50, and off paper, at 100. Looking out between the two you could see puffs of smoke about 4 feet up, 75 or so yards out when the rifle fired. Its was a neat thing to see. Im sure military bullets would be fine even at the speed because the specify jacket material and dimensions. Commercial bullets (especially the cheap ones I saw this happen with) are looser. I agree the military specifies to avoid them in the A2 because of accuracy/wear.

It was standard practice in the late 80's and early 90's (during the changeover period from the A1 and older carbines to the A2 and newer carbines like the MK727, which eventually became the M4) when there was an excess of M193 55 grain ammo, and a shortage of M855 62 grain ammo- for the 55 grain ammunition to be issued for use in the older weapons for stateside training. While there was some effects in regard to accuracy, it was mostly exhibited during 25 meter zeroing. To my knowledge, they caused no excessive wear on the weapons- after all, the chamber pressures and velocity are lower. As for cheap commercial bullets disintegrating in flight, all I can say is that you get what you pay for.

 

[ColoradoMinuteMan]

Actually the chart shows they experienced <2 MOA w 75 grain projectiles, so I don't see the delta you're taking about w75. You're right it does show deviation from your experiences w 77. Not sure if you actually read the research or just looked at the chart, but the research makes no claim of being definitive or representative of everyone's experiences.

 

[FL-NC]

Actually the chart shows they experienced <2 MOA w 75 grain projectiles, so I don't see the delta you're taking about w75. You're right it does show deviation from your experiences w 77. Not sure if you actually read the research or just looked at the chart, but the research makes no claim of being definitive or representative of everyone's experiences.

I stand corrected regarding the 75's, but stand by my statement on the MK 262 (77).

 

[ColoradoMinuteMan]

Fair. They don't specify the 77gr ammo used, but they describe it as match 77gr. It could be Mk262 which uses Sierra MK 77gr HTBT, but it also could be Sierra TMK which is a longer projectile. Since length is the largest factor driving stability in relation to a given caliber and twist rate, a variation in specific projectiles can make a significant difference even with the same weigh bullet.

I think the main point of demonstrating the 77gr may not perform well in a 1:9 isn't to be any sort of hit on 1:9 or proof that it won't work, but rather to help set expectations with those who're assessing options. If the plan is to shoot 80% 75s and 77s, you might want to find something that is designed to regularly perform well with those projectiles. If you're shooting <50 grain you may want to find something optimized for that. You never know what changes or variations there may be from cartridge to cartridge and if I prefer heavy for caliber projectiles, I don't want to be limited in selection because of bullet length, etc. Even worse, I intend to shoot mostly 75gr. Bullets but I get a 1:9 twist rate based on a recommendation from someone who beats the odds getting great accuracy with 77, only to find out, I didn't beat the odds and now have a less than optimal performance.

The right answer is to figure out what projectiles you want to be shooting, and purchase something that has a known reputation for performing to your standards with those projectiles.

 

[tactikel]

My 16" carbine with a 1:9 twist shoots 69 gr SMKs with outstanding accuracy. If I ever needed to shoot anything heavier I'd install a 1:7.

 

[RussellC]

My loads were not the fast. Well, I didnt actually chrono them but the load table predicted 2800-2900 fps. Also, they were Sierra match kings, I suppose a higher quality bullet, or at least more expensive. I did see the results of an accidental discharge with a 55 grain Hornady FMJ inside a house, which went across a dining room, across the kitchen and through a double pane patio door and then in the ground. (At least he had followed the other "safety" rules) I was surprised to find most of the copper jacket laying on his kitchen floor. I dont know if it exploded, or the glass skinned it and it bounced back or what. There was a hole through the glass, then through a screen. The hole through the screen was surrounded by 6 or 8 small holes, which I assumed was from glass fragments, a few were still on the screen. It was only about 15 yards from point of fire to the screen. Is it possible the lead core blew out of the jacket? I have no idea what sort of load or ammo, just that it was a Hornady 55 grain FMJ bullet.

My holes in target at 100 yards were round...

Russellc

 

[MachIVshooter]

This. Unless you want to shoot the heavies, say 70 Gr and up, 1 in 9 is good to go.

This. I never run anything heavier than that, and prefer 1:8 or 1:9 barrels to 1:7, because I like to run 40 grainers frequently.

 

[jmorris]

A 1:9 can shoot bullets that will come apart in a 1:7 due to centrifugal force. Not as common these days but back when there were more bolt action .223 caliber rifles 1:14 twist was more common and thin jackets were ok but Ina 1:7 only fragments would hit the target even as close as 25 yards.

 

[Klint Beastwood]

jacket separation and even disintegration of the bullet is possible when over twisted on some bullets. This however isnt truly a stability issue as much as it is a bullet design issue not being used correctly. There isnt really an issue. A 308 in most grains can be used from a 12 twist to 9.5 pretty reliably. I've heard of some more professional people getting away with 1:8 as well, using monolithics

 

[mjsdwash]

This. Unless you want to shoot the heavies, say 70 Gr and up, 1 in 9 is good to go.

yup, but i bet 99% of the 223/5.56 market is made up of people shooting 200 yards or less, and 90% is people shooting at junk in gravel pits and ranges.

 

[Walkalong]

The most I have these days is 300 yards at the range. I tried and quit on the heavy bullets for a couple of reasons, and that was one of them.

 

[mjsdwash]

The most I have these days is 300 yards at the range. I tried and quit on the heavy bullets for a couple of reasons, and that was one of them.

yep, still have half my only box of 69gr matchkings. They worked great, but I moved too far from the 600 yard range. Now I can do 200 meters at best and I don't shoot nearly good enough to stop using the 55's.

 

[HankC]

I don't shoot long distance that need heavy bullets or like military shoot tracer rounds, I prefer 1/9 in AR with 16" barrel. When I buy a 223 bolt gun with 22" barrel, I prefer 1/12 twist. High twist rate theoretically wear barrel faster, spin bullet faster than needed also mean potentially less accurate while bullet balance/concentricity would become more critical. If one day I were going to shoot long range, I would pick my 308s anyway.

 

[1stmarine]

hi,
The grain of a bullet might indicate the twist but the required twist is not determined by the grain but the bullet length, speed, construction, etc..
1:9 is the standard for popular ammo like typical 55, 62 and 70gr as soon as the 70g is not a solid or some other bullet like the Barnes 70gr TSX that needs a 1:8.
The 1:9 might stabilize some of the high performance 75gr-77gr bullets but it is not the ideal choice..
The 1:8 is the standard in national match and it is perfect for most high performance ammo including 75gr-80gr low drag bullets. Many times these are the bullets
that win competitions because they buck the wind a lot better. Some high performance ammo and solids will also require the 8 twist.
The 7 twist is the military standard. It is required to properly stabilize tracer ammo and can also be used to stabilize some of the longest bullets like some solids
and VLD projectiles in the 70gr-80gr range. But most of those require single feed or VLD single stack magazines along with match chambers.
There are some match makers who make 7.5 and 7.7 twist for match purposes.
It seems to me the 1:8 twist is perfect for the AR platform but many folks might never shoot match ammo or long hunting solids trough the AR so they will be ok with the 1:9.

I am also saying because if you look at many calibers like the 6mm, 6.5m, 7mm the 1:8 will be perfect too for some of the best bullets those have to offer and falling into
the same bore to length relationship.

Personally if I cannot choose 1:8 then I rather have 1:7 vs. 1:9 for a general purpose even if I don't have an immediate need. Cannot be sure I will never do that so
better to have it and don't need it than need it and don't have it.

IMO there are three fashions that do not make much sense in the AR15 carbine:

- 1:8 should be the standard for the typical do it all carbine. I explained why above.
- Mid length should be the standard port for the average 16" carbine. Less port pressure, longer tube, less heat on the bolt, more versatile.
- The tacticool M4 style profile should be replaced by a straight cut stronger barrel. The M4 style cuts nor the carbine port serve any practical purpose because all they
do is to compromise the minor diameter ahead of the chamber where the greatest part of the heat collects and nobody will be attaching a grenade launcher bracket
so we don't need the those cuts ahead of the gas block nor the gas block there.

 

[Walkalong]

The tacticool M4 style profile should be replaced by a straight cut stronger barrel.

Amen brother....