5.56 twist rates..... 1 in 7" vs 1 in 9"

[Runningman]

Is it true that during USMC testing back in the 1980s for the A2 using the 5.56, group sizes opened up by like 90% after only 6,000 rounds using the 1 in 7" twist barrel. Compared to only slightly degraded accuracy after 10,000 rounds using a 1 in 9 twist barrel. Thought I read or heard something like this years ago. Feel free to correct me if I remembered it wrong.

From what I remember the military settled on the 1 in 7" twist in the end only because of the than new burning requirement time for the 5.56 tracer round.

 

[FlyinBryan]

dont know.

i do know the tracer rounds are a big reason the 1 in 7 twist is used.

it does make sense the a 1/7 would degrade before a 1/8 or 1/9.

i think the top ranks in service rifle competition use a 1/8.

ive had 1/8 and 1/9 twist only ar's and ive been more accurate with the 1/9 although to be fair i think the 1/9 barrels ive had from bushmaster have been a little higher quality that the 1/8 colts ive owned.

then again, the heaviest bullets i ever load are in the 68 grain range.

 

[Javelin]

The 1:7 stabilizes heavier bullets (75gr.+) in shorter barrels.

The 1:9 does great with lighter bullets such as the 55gr. and 62gr. which is what most folks shoot. Some folks shoot 1:12 twist if primarily using 55gr and lighter bullets.

It is worth mentioning that a 16"+ barrel with 1:9 twist will stabilize up to 75gr. as the 1:7 is primarily needed for 14.5" and under to reliably do so.

 

[FlyinBryan]

It is worth mentioning that a 16"+ barrel with 1:9 twist will stabilize up to 75gr.

hmmmm

i have never heard this.

maybe i should pick up some 75's and see what they will do.

 

[sarduy]

i dont know... i shoot from 55gr to 75gr at 100 yards any are about the same... 1-2 moa.

 

[Jeff White]

Is it true that during USMC testing back in the 1980s for the A2 using the 5.56, group sizes opened up by like 90% after only 6,000 rounds using the 1 in 7" twist barrel.

No it's not completely true. The Marines only did a 6000 round endurance test and the group sizes went like this:

Start-------------------3600 rounds-------6000 rounds
27.43 cm---------------31.23 cm ----------62.23 cm

The poor grouping was attributed to the XM855 ammunition, not the twist rate. M855 bullets have proven difficult to manufacture since they were designed.

1/7 twist barrels can go a lot more rounds before the they begin to wear enough to seriously degrade accuracy. My Colt R6920 has over 12K rounds through it and still shoots as tightly as it did when it was new. It has a 1/7 twist.

From what I remember the military settled on the 1 in 7" twist in the end only because of the than new burning requirement time for the 5.56 tracer round.

The M855 and M856 rounds were developed for the SAW program. Since it was desirable to utilize the same ammunition in both the product improved M16 and the SAW, it was necessary to adopt the 1/7 twist on what became the M16A2. And the 1/7 twist was adopted for the SAW because the FN L110 tracer round (which became the M856 tracer when adopted by the US forces) is significantly longer then the ball round. And the round id longer because of the tracer burn requirement for the SAW.

The 1:9 does great with lighter bullets such as the 55gr. and 62gr. which is what most folks shoot. Some folks shoot 1:12 twist if primarily using 55gr and lighter bullets.

Actually it's more about bullet length then weight. There are 60 grain bullets out there that shoot just fine in 1/12 twist barrels. The steel penetrator and the air pocket in the tip of M855 requires the bullet to be longer to get it heavy enough to have the desired terminal effects at range.

It is worth mentioning that a 16"+ barrel with 1:9 twist will stabilize up to 75gr. as the 1:7 is primarily needed for 14.5" and under to reliably do so.

I think this is a barrel by barrel call. I've seen 16" 1/9s that shot 75 grain bullets really well and I've seen some that won't.

 

[Ned Kelly]

1 in 9 twist rate is the best all around. Slow enough not to make your varmint bullets fly apart out of the barrel, fast enough to stablize ss109 rounds.

 

[kwelz]

1 in 9 twist rate is the best all around. Slow enough not to make your varmint bullets fly apart out of the barrel, fast enough to stablize ss109 rounds.

I am sorry but this is wrong. 1/9 has trouble with heavier bullets. anything above 69 Grain is going to be a problem. 1/7 is a superior twist for a wider range of common bullet weights. the only thinkg that it might have trouble with is something like a 35 grain Vmax.

As far as barrel wear goes I can see the argument that 1/7 will wear faster. However when is the last time you saw a burned out AR barrel?

 

[MutinousDoug]

Actually, I have a "burned out" AR barrel down in my basement right now. It was pulled off my High power gun last Fall after 5086 rounds down range.
It is a 1 in 8" Olympic barrel that, when new, shot 69g Sierras into under 2" at 200yds (aperture sights). When I pulled it, it was shooting 10" groups at that distance.
If anyone would like to see if they can get more mileage out of it (or would like to see what a "burned out" barrel really looks like), I'd consider all/any offers. Otherwise it's going to become a SS tomato stake.

 

[Ned Kelly]

I am sorry but this is wrong. 1/9 has trouble with heavier bullets. anything above 69 Grain is going to be a problem. 1/7 is a superior twist for a wider range of common bullet weights. the only thinkg that it might have trouble with is something like a 35 grain Vmax.

As far as barrel wear goes I can see the argument that 1/7 will wear faster. However when is the last time you saw a burned out AR barrel?

52 grain match bullets will break up out of a 1 in 7 twist barrel.

1 in 9 will stablize bullets weighing 62 grains (ss109)

For hunting the .223 is best used against varmint animals. Bullets between 50 and 63 grains are usually the best all around for killing varmints.

We are talking about the mini-14. The military will never use this gun. Law enforcement might.

55 grain ammo is cheaper for an agency to purchase than 62 grain ammo.

The mini 14 was supposed to target civilian shooters who wanted a truck gun. Hence the name Mini-14 Ranch rifle.

The 1 in 7 twist rate costs ruger a lot of lost sales due to the bad reputation of the ruger accuracy wise.

Using 69 grain bullets I was able to shoot 1 and a half inch groups with a standard ruger ranch rifle. 69 grain match is not even close to the performance of lighter weight hunting rounds when killing varmints.

So I am not wrong when discussing the main purpose of the gun in the civilian and law enforcement sector.

I also don't see law enforcement needing a 1 in 7 to stablize tracer rounds either.

 

[benzy2]

You are talking about the mini 14. I don't think anyone brought that up but you just now? I thought we were talking AR/M16 barrels?

 

[briansmithwins]

Hmmm, I guess I had better not tell my 45gr and 50gr varmint bullets that they are supposed to 'fly apart' when I shoot them out of either the 16" or 20" 1/7 barreled ARs.

I get better accuracy out of 75gr PPU with those rifles than with either XM193 or 55gr PPU, but we're talking 1.25MOA vs 1.75-2MOA with the lightweight guys.

Buy the barrel you want and figure out which ammo works best for your setup. 1/7 has been fine for me. BSW

 

[spiroxlii]

I have a Colt LE6920 with a fairly short 1:7 barrel. I realize that with this twist rate, I COULD stabilize heavier bullets, but most of the rounds I've put through it have been 55gr M193-style ammunition.

I'm not a competition shooter. Honestly, I never wanted an "evil black rifle" until recently. I bought it just so I'd have one. This thing is a range toy for me. My longest shots have been about 50yd, and I never bothered to measure the groups scientifically. All I know is that I make one big ragged hole in the paper. If my target were a hog or the intended target of the M4/5.56 combo (an enemy combatant), then this rifle would probably be accurate enough, and I've never had a round fragment upon leaving the barrel.

 

[Ned Kelly]

I thought this was the ruger thread. If you intend on using it in combat than 1 in 7 is the twist rate you want.

Not if you are intending to use it to go hunting with lighter bullets.

Hmmm, I guess I had better not tell my 45gr and 50gr varmint bullets that they are supposed to 'fly apart' when I shoot them out of either the 16" or 20" 1/7 barreled ARs.

http://community.discovery.com/eve/forums/a/tpc/f/9741919888/m/74219531101/inc/-1

There are bullets out there that are too fragile for velocities above a certain level and cannot be fired from twist rates faster than a certain rate.


Some of the Sierra 22cal offerings will literally "blow apart" from a combination of too much velocity and too much rotational force imparted by the rifling.

Typically, light varmint bullets in 22cal are intended for slow twist rates of 1:12-1:14. This is where you often find the 22-250s that are intended for light weight bullets.

When you push a 40grn bullet at near 4100-4300fps, a twist rate of 1:9 may be too much for the bullet to handle.


As for "streaks to the target". Some of it can be attributed to vapor trails as the bullet cuts a path through the air, basically you see the mirage or difference in density of the wake of the bullet. A trace to the target is often used by spotters/snipers or in competition circles to spot the bullet's flight to the target.

Other times there's what is sometimes referred to as a blue or grey streak, may have something to do with moly bullet coatings fired in very fast 22cal chamberings.

As usual Hillarity is correct. The rotational speed, and not the linear speed is what causes the failure due to the centrifugal force. Note that failures only occur in fast twist barrels. The grey streak is typically the lead core breaking apart. This tends to occur with very lightly constructed varmint bullets (very thin jackets, and soft lead). For example a bullet fired from a 1/12 twist barrel (standard varmint rifles)at 3200 fps will leave the barrel spining at 192000 RPM. The same bullet fired from a 1/7 twist barrel (standard for M-16) at the same velocity will be spinning at 326400 rpm when it leaves the barrel.

I have quite a bit of experience with the 1 in 7 twist rate when I use to mess with mini-14s. I know for a fact that thin jacketed light weight bullets WILL break apart if they are going to high of a velocity for the barrel spin.

I have owned several ar15s that used the 1 in 7 but never fired light weight thin jacketed bullets through them based on my experience with the Rugers.

 

[Runningman]

As far as barrel wear goes I can see the argument that 1/7 will wear faster. However when is the last time you saw a burned out AR barrel?

I know my AR DCM 1 in 8 twist barrel started shooting very large groups around 4400 - 4600 rounds. Bore scope showed severe throat erosion. However, this barrel was not chromed lined (usually most Service Rifle competition AR barrels are not).

Jeff White thanks for the info.

 

[Jeff White]

You're welcome, you can find more data on the development of the M16 in The Black Rifle by R. Blake Stevens and Edward C Ezell. Collector Grade publications.

Chrome lined bore last much longer then non chrome lined bores. Unlike larger weapons like mortars, howitzers and tank cannon, the military keeps no data on how many rounds are fired through small arms unless they doing a test. Small arms are gauged annually and if a barrel is out of spec, it will be replaced. They use a barrel erosion gauge and a barrel straightness gauge.