[eastwood547]

I am new to the AR. I purchased the RRA operator elite 2 and it has the 1:9 twist barrel. In the next couple months I will start putting together my list to start my first complete build. I was told the 1:9 was great for the typical 55 to 64 grain rounds which is all I ever shoot (cheap and available) so why the 1:7? It seems no I read more it's all about mil spec and the 1:7. What are the benefits?

Also, I apologize if this has been posted before, but I couldn't find it (maybe i'm just dumb but hey). Thanks for any help on this matter.

[rbakis]

The short answer is 1:7 twist provides the stability for heavy/long bullets (77 grain/tracers) to have long range accuracy. Most 1:9 barrels will shoot up to 69 grain bullets accurately. Go to 1:7 if you want the flexibility to shoot the 77's. The 1:7 barrels will shoot 55's well but may not be as accurate as a 1:9 shooting 55's.

[TomJefferson]

What rbakis said is a good rule of thumb, however I think a rough understanding may help some.

Every bullet has an ideal speed and spin for it to fly best. It will fly well within a range of ideal. All a twist does is get the bullet spinning. How fast it spins then is a matter of how fast it goes down the barrel. For a given powder charge, the longer a barrel the faster the bullet will travel, so the faster the spin. It follows then, the shorter the barrel, the more desirable a faster twist for a given bullet.

This takes us into the M16 development, which originally was for a 55 grain bullet and a 20" barrel with a 1:14 twist. These were amazing shooters in their day and qualification ranges were out to 650 meters. Over the years, the military first tested under different conditions (for example, first twist change was to go faster because of cold climate dense air conditions) and then started using shorter barrels and heavier bullets. As it did, the effective range of the rifle was for all purposes pulled in. Now this is not as bad as it sounds. The M16 has spent its history pretty much up against the 7.62X39mm AK/SKS round which ballistics are about twice as much drop over a given distance.

It shouldn't surprise you then, the first production high volume version to get a 1:7 twist was the M4 with its 14.5" barrel. As M16 and AR15 variants increased so did bullet selection to where now I daresay there's more bullet selection for this caliber than any other. They range from very fast light bullets 40 grains and under to heavy long bullets over 70 grains. The two most common bullets we all shoot are 55 grain and 62 grain. The 1:9 twist then is compromise twist which will shoot the two most common rounds across most barrel lengths.

Hope this helps.

Tj

[helotaxi]

Unless you plan on shooting bullets that have to be single loaded because they are too long to fit in the magazine or 64gn tracers in the Arctic cold, your 1:9 barrel will suit you just fine.

[eastwood547]

Thanks guy's, and good write up Tom! I like learning about my rifle. I think I would rather be able to shoot my normal everyday rounds consistently than to be able to shoot the long heavy rounds (although would be fun) I will consider these variables when it comes to the barrel purchase. Thanks again!

[GLShooter]

Thanks guy's, and good write up Tom! I like learning about my rifle. I think I would rather be able to shoot my normal everyday rounds consistently than to be able to shoot the long heavy rounds (although would be fun) I will consider these variables when it comes to the barrel purchase. Thanks again!

I solved this little dilemma the other day. I ordered a 1:8!!

The plan is starting at 600 yards and then trying a few at 1000. It should be a great learning curve and will let me shoot in the basic Field Target Class and when the 6 MM gets here I can have an Open gun for the long targets.

Greg

[Paratrooper82]

Unless you plan on shooting bullets that have to be single loaded because they are too long to fit in the magazine or 64gn tracers in the Arctic cold, your 1:9 barrel will suit you just fine.

Please explain further.... I have carried M193, and fired it from both M-16A2's, and M4's out of standard issue mags. It was never to long..... The M193 is what we use for SDM...

[rifleman2000]

Please explain further.... I have carried M193, and fired it from both M-16A2's, and M4's out of standard issue mags. It was never to long..... The M193 is what we use for SDM...

I think he is referring to handloaded target rounds that are too long to feed from a magazine.

[Paratrooper82]

I think he is referring to handloaded target rounds that are too long to feed from a magazine.

Thanks Rifleman. Airborne!

[helotaxi]

I think he is referring to handloaded target rounds that are too long to feed from a magazine.

Correct. Specifically the 75 and 80gn A-Max and 80gn SMK as well as some of the Berger VLDs. The 75gn Hornady TAP works just fine in a 1:9. Mk262 would be the only mag length load that would be iffy, and it isn't exactly commonplace.

Don't think that accuracy will be compromised with shorter lighter bullets in the 1:7 barrel either. That is 99% internet myth based on anecdote not data. Brian Litz discusses it at length in Applied Ballistics for Long Range Shooting. It you're talking benchrest accuracy, there might be a difference that could make a difference. From a practical standpoint with a normal rifle, the difference will be at the noise level, lost inside the standard deviation for your group sizes.

Barrels of similar construction and quality will shoot similar bullets roughly the same regardless of rate of twist. I've shot bullets ranging from 36gn through 75gn from my 1:7 carbine and the quality of the bullet determined the accuracy, not the bullet length/rate of twist relationship. Cheap bullets (particularly FMJs with their relatively inconsistent heels) are susceptible to balance issues and a high rate of twist can aggravate that slightly, but those bullets are far from accurate anyway; like 3-5" groups at 100yds. Quality bullets will shoot well from quality barrels. Buy a quality barrel and feed it well and it will do right by you.

Edited by helotaxi, Feb. 09 2012 - 09:31 AM.

[Paratrooper82]

Correct. Specifically the 75 and 80gn A-Max and 80gn SMK as well as some of the Berger VLDs. The 75gn Hornady TAP works just fine in a 1:9. M193 would be the only mag length load that would be iffy, and it isn't exactly commonplace.

Don't think that accuracy will be compromised with shorter lighter bullets in the 1:7 barrel either. That is 99% internet myth based on anecdote not data. Brian Litz discusses it at length in Applied Ballistics for Long Range Shooting. It you're talking benchrest accuracy, there might be a difference that could make a difference. From a practical standpoint with a normal rifle, the difference will be at the noise level, lost inside the standard deviation for your group sizes.

Barrels of similar construction and quality will shoot similar bullets roughly the same regardless of rate of twist. I've shot bullets ranging from 36gn through 75gn from my 1:7 carbine and the quality of the bullet determined the accuracy, not the bullet length/rate of twist relationship. Cheap bullets (particularly FMJs with their relatively inconsistent heels) are susceptible to balance issues and a high rate of twist can aggravate that slightly, but those bullets are far from accurate anyway; like 3-5" groups at 100yds. Quality bullets will shoot well from quality barrels. Buy a quality barrel and feed it well and it will do right by you.

Ok sorry Helo! I have seen this posted before and wondered. You can buy civialian XM193 ammo. Now it is not the military surplus however, if it shoots like 193, looks like 193 its 193.... In my expirance M193 is an outstanding bullet to shoot from a issue M16A2 or M4. I have used it on targets out to 700 meters with great accuracy and repeated sussess!

[TomJefferson]

No, the internet myth is the only twist to have is 1:7 like AR and M16 manufacturers are ripping off the world with different twists.

The physics are what they are and you don't gain accuracy by accepting one more variance among others but by eliminating variance.

Tj

[helotaxi]

What's the variance that you speak of? It isn't like the barrel changes rate of twist between shots. Plenty of high-power shooters use a rifle with a (sometimes very) fast twist barrel and have no issues with shooting lighter bullets for the shorter stages. The physics are what they are and they are very well explained in the book mentioned above. His determination based on the physics is that it doesn't matter. The bullet is either stable or it isn't. Once it is to the point of being stable, spinning it faster offers no further benefit but also doesn't do anything detrimental either unless the bullet is of poor quality and unbalanced to begin with. At that point it might open a group up slightly.

As far as the M193, I was thinking Mk262 (Black Hills loaded 77gn SMK) not the old 55gn FMJ load. Just about any rifle and any AR will shoot M193 just fine. The 77gn SMK will load to mag length but is hit or miss with stability (mostly miss from what I've seen) with 1:9 twist barrels. It is not that common. M193 (and clones) are about the most common round out there. I fixed my previous post.

[Paratrooper82]

Helo,

Thats what i get for typing faster then my mind thinks... Yes its the MK 262. I have never had a problem with it out of military weapons. The SDM (Squad Desinated Marksman program) takes a M-16A2 rebarrels it and replaces the trigger. I have seen, and done great things with this set up.

[TomJefferson]

What's the variance that you speak of? It isn't like the barrel changes rate of twist between shots. Plenty of high-power shooters use a rifle with a (sometimes very) fast twist barrel and have no issues with shooting lighter bullets for the shorter stages. The physics are what they are and they are very well explained in the book mentioned above. His determination based on the physics is that it doesn't matter. The bullet is either stable or it isn't. Once it is to the point of being stable, spinning it faster offers no further benefit but also doesn't do anything detrimental either unless the bullet is of poor quality and unbalanced to begin with. At that point it might open a group up slightly.

As far as the M193, I was thinking Mk262 (Black Hills loaded 77gn SMK) not the old 55gn FMJ load. Just about any rifle and any AR will shoot M193 just fine. The 77gn SMK will load to mag length but is hit or miss with stability (mostly miss from what I've seen) with 1:9 twist barrels. It is not that common. M193 (and clones) are about the most common round out there. I fixed my previous post.

If that's what he determined, he's wrong. Every bullet has an ideal speed and spin to fly its best. Exceed either of those parameters it flies worse though within a range well enough.

Ideal flight is a given ratio of spin to speed for a given bullet. As a bullet travels it naturally loses speed and the ratio then acts more like a square root scale rather than a lienier scale. To put that in plain English, as the bullets speed decreases the ratio of spin to velocity changes. Air resistance is more on the bullets flight path than on the spin. Too much spin, a bullet yawhs sooner.

I can post a smooth bore does fine and dandy from here to the wall, but it doesn't change the fact a rifled barrel is more accurate anymore than saying at 100 yards it doesn't matter changes physics.

Distance simply is part of the equation of accuracy.

The myth is all barrels should be 1:7 and the industry is FOS for having other twists. It may suit someone's shooting style but not everyone shoots from here to the wall.

Keep in mind as per my first post, all a twist does is get a bullet spinning. How fast it spins then depends on how fast down the barrel and therefore barrel length is part of the equation. To find ideal travel then for a bullet, the slower the twist for the longer the barrel and vice versa.

Tj

[TomJefferson]

BTW, Another good discussion is how hollow point boat tails fly. They're a little different in that due to the aerodynamics of the bullet design, they stabilize more at distance.

I like to think of them like the old German V2 rockets that wobbles as it takes off then falls into a straight flight path.

If you follow my last post then, spin plays a big part in when they stabilize and because of the Aerodynamics a real balancing act of bullet variances, weight and shape, of consistent aiming points.

I marvel at the idea somebody came up with it as an idea. i understand how it works, its just amazing to me someone came up with the concept to achieve such a goal. That was radical new thought in its day.

Tj

[mtrmn]

You might want a little faster-than-9-twist for some of the heavier Barnes bullets because they are longer due to the lighter-weight material they're made from. Other than that, a faster twist than 1/9 would not benefit me in the least as far as I know.

[eastwood547]

Ok, one of the reasons I started this thread is that a friend of mine said I got ripped off by buying a RRA Operator Elite when I could of bought a Spikes for less. His argument was that the Spikes was "mil spec" and had a 1:7 twist which was far superior to my 1:9 commercial gun! Just kinda annoyed me and I wanted some answers before I started a build. Thank you for all your replies as I have learned a lot, and by the way I absolutely LOVE my RRA! Nothing against Spikes I just liked the RRA, but maybe it is a better rifle for less IDK.

[helotaxi]

BTW, Another good discussion is how hollow point boat tails fly. They're a little different in that due to the aerodynamics of the bullet design, they stabilize more at distance.

Theory not backed by physics assuming that the spin imparted on the bullet is sufficient for the bullet length from the muzzle, unless it went into the rifling slightly sideways which would cause it to exit slightly sideways and then correct back to a concentric spin once it no longer had the external constraint of the barrel keeping it yawed to its center of gravity. This is why VLD designs are often more precise when jammed into or at least touching the lands. It minimizes the chances and degree of the bullet hitting the lands with some amount of yaw. Same idea as minimizing bullet and case runout for high precision ammo.

I like to think of them like the old German V2 rockets that wobbles as it takes off then falls into a straight flight path.

The rocket wobbles because unlike a bullet, it requires the aerodynamic forces of the fins to stabilize it and they do not provide adequate stabilizing force until a certain velocity is reached. As the rocket comes off the pad it has to accelerate to that velocity. From leaving the pad until the critical speed for fin stability is reached, the rocket oscillates back and forth between the points where the fin angle of attack and relative wind creates enough force to counteract the yaw moment and correct it back the other way. The amount of yaw that this requires decreases with velocity until the fins are in equilibrium with each other and the aerodynamic upset forces that are trying to tumble the rocket.

If that's what he determined, he's wrong.

I guess the you should tell Berger that their Chief Ballistician doesn't know what he's talking about.

To put that in plain English, as the bullets speed decreases the ratio of spin to velocity changes. Air resistance is more on the bullets flight path than on the spin. Too much spin, a bullet yawhs sooner.

Based on what force? The only upset forces on the bullet are gravity and the torque generated by the difference in the aerodynamic center and the center of gravity of the bullet. Spin counteracts both of those forces. Gravity is constant and a bullet of a given length only needs to spin above a certain RPM, regardless of velocity, to negate the effects of gravity on its stability. The aerodynamic upset force is proportional to bullet velocity. As the bullet velocity decreases, this force decreases and the RPM required to counteract it decreases as well. Since the RPM decays at a much lower rate than the velocity, a bullet's stability increases as it flies downrange. More RPM increases stability. There is a slight precession caused by any asymmetry in the bullet itself that is amplified at higher rates of spin and that is the only negative effect of spinning a bullet faster than absolutely necessary to achieve stability. That precession is present regardless of the rate of spin though and is essentially negligible with high quality bullets.

The only other way that an otherwise stable bullet tumbles is if an unequal force acts on it enough to push it out of its equilibrium. That either means that the bullet has to hit something or there is a change in the flight regime such as entering and passing through the trans-sonic velocity range. How the bullet behaves in either of those circumstances are much more a factor of the design of the bullet than the rate at which it is spinning.

No personal offense intended, but I'm much more inclined to take the word and corresponding analysis of a guy who actually studies, designs and tests bullets for a living...if you haven't read his book, I would highly recommend it. I'm about to buy the second edition if you want my copy of the first to look over.

[TomJefferson]

Spin has nothing to do with gravity, bullet drop. Its not an anti-gravity device. Nor does it grossly impact the farther a bullet travels the slower it becomes. Its entirely in keeping it stable not wobbling so taking a new course of direction. Yes, theoretically if you had a perfect uniform bullet excess spin would have no impact on its stabilization, however as a bullet slows in forward velocity, how important that is increases because the ratio to velocity becomes greater. That imperfection then becomes like a tennis shoe in a washing machine. Nobody makes a perfect bullet.

This is why bullet shapes have changed over the years. Like the V2 rockets spin, the hollowpoint pushes air around and over the body effecting the vacuum lift created by the boat tail. Like those fins, those feature boattail and hollow point, have an ideal velocity where they work best. Where depends on the design, velocity, air density, etc. but if you have ever shot a 69 grain Sierra Matchking, you'll notice it can actually group better farther down range than in. The design makes it more like airplane than a rocket. This impact ballistic coefficient or how well a bullet flies.

One twist fast as you can for all bullets is a hard sell. The industry really is trying to do their best they can not just scam potential customers.

I doubt very seriously your author is trying to make that point. If he is, he missed some real basic engineering courses in school. He may be using minimization what he considers the importance which is basically a preference but the principles are pure physics. To make that assumption one has to limit range for certain bullet types.

Bullet drop is a very known factor. We have calculators all over the place, free on the web, etc. If the 1:7 twist was the cure all, we'd see high optics on M4s and call them sniper rifles. Shoot your 1:7 with 55 grain out some distance, calculate the drop. See how far it can go. Now do the same with a slower twist. You'll find, you can hit things a lot farther out with the slower twist. This is something you can experience it first hand, not read it in a book or in a post on the internet.

Despite having parts I designed in outer space, I'm not going ask you trust me, throw our my degree, or ask you to read something I published, I'm only going to ask you to shoot that gun and find out for yourself.

1:7 in an AR barrel came about as a fix to a problem not the cure all discovery that the world is slow to get on board. Changing that rifling around to see what works best across different barrel lengths and bullet sizes has been around since rifling has, the early 19th century.

Tj