Ok guys, really need some help. After a year I'm in the final stages of saving money for an RRA Entry Tactical and I have gotten a bunch of help here. People have been great at dealing with my FNG questions. So here, I hope, is the last one: Do I really need a 1/7 twist barrel? I want to swap out the standard 16" Wilson that comes with the RRA and replace it with a 14.5" with perm. attach FH. My plan until now has been to get a Bushie 14.5" with a Phantom already on there. However, many people really seem convinced that 1/7 is the best way to go. Problem is that if I go that route I will spend quite a bit more: an LMT/Colt/CMMG 14.5" 1/7 seems to be going for more like $300-$350. My local dealer can get me a Bushie 14.5" w/Phantom for under $200. This isn't a huge difference, but for a college student it is something.

So school me folks. If I really need a 1/7, please explain why now. Thanks, again, for all the help in the last month guys!

You do not need 1:7. The only advantage would be stabilizing heavier 5.56 rounds. I have a 1:9 Bushy and it keyholes ~50% of my shots if I use heavy rounds.

Other than that there is absolutly nothing 1:7 would do better than 1:9. Use 1:9 with 55gr. Xm193 and you will tear some shit up.

www.ammo-oracle.com/

and if that doesnt answer your question just get the 1x9 from bushmaster and have fun, upgrade later if you feel the need.

Unless you are going to shoot the real heavy stuff, like 75 or 77 gr., you don't need a 1/7.  1/9 works for most people.

1/9 works up to 75 grains if it is a true 1/9.  BTDT.

1/8 works up to 80 grains.  BTDT.

The .mil sepcified 1/7 to stabilize the 5.56 tracer bullet, which is a very long bullet.  Contrary to popular belief, itr is bullet bearing length, not weight, what determines what the twist needs to be.  In general, the heavier the bullet, the longer it needs to be.

The difference in required twist between the Hornady 75 and the Sierra 77 is interesting.  You would think that 2 grains would not make any difference.  And they don't.  It's the fact that the Sierra 77 has a longer bearing length than the Hornady that makes them borderline in a 1/9.

Long story short..........the faster twist rate delivers a heavier (more deadly) round more accurately  

Same reason you should want it.

Well... dont take this wrong, but you want a 1:7 because that is what other people say you want; the same reason you want to cut 1.5" off of your barrel and are getting, the same reason you need th GL cut, the same reason you are going with 7.5 GS instead of the much better 9.5 GS -- The rifle you think you want is a choice of form, not function... so why rationalize this when so many other things are suspect?

BTW: as mentioned, ROT does not depend on bullet weight, but it does depend on velocity; 16" 1:9 barrels are smoking rigs with TAP and despite what some people think, there is a downside to running a twist rate faster than you need... reduced MV for one thing... faster throat erossion.

1:8 will shoot anything you can fit in an AR mag just fine (and then some). If you like RRA, look at the elite tac with the 1:8 SS... or call ADCO and have them build you the exact upper you want.

Please understand, I am not bagging on your choices at all, I tell people this all the time: If you really *need* a gun then get what you need; If you just *want* a gun, then get what you want!

Merry Christmas!

Gunzilla always sets it straight.

Quoted:ROT does not depend on bullet weight, but it does depend on velocity; 16" 1:9 barrels are smoking rigs with TAP and despite what some people think, there is a downside to running a twist rate faster than you need... reduced MV for one thing... faster throat erossion.

ROT is marginally dependent on MV.  Bullet bearing lenght is the overwhelming factor at play.

Faster twist rates reduce MV? .  You had best be showing me the results of a designed experiment matrix (Taguchi, anyone) before I can believe that.  Same for faster throat erosion.  Throat erosion is a function of flame temperature and duration.  Period.

But then, ARFCOM dogma sometimes has nothing to do with reality .

Quoted: Quoted:ROT does not depend on bullet weight, but it does depend on velocity; 16" 1:9 barrels are smoking rigs with TAP and despite what some people think, there is a downside to running a twist rate faster than you need... reduced MV for one thing... faster throat erossion. ROT is marginally dependent on MV.  Bullet bearing lenght is the overwhelming factor at play. Faster twist rates reduce MV? .  You had best be showing me the results of a designed experiment matrix (Taguchi, anyone) before I can believe that.  Same for faster throat erosion.  Throat erosion is a function of flame temperature and duration.  Period. But then, ARFCOM dogma sometimes has nothing to do with reality .

Well... you answered some of your own questions, I will try to make this simple. Stability is dependant on rotational speed (in a given air density), this RPM is determined solely by the ROT and the velocity; check out what sort of RPM difference you get with a 500 fps change (using whatever ballistic development software you use) and then see how that change in speed effects the required ROT to reach a stability factor of 1.5 (why go higher? location, location, location). We all know that it is the finite length of the bullet that is the major consideration when  determining the rotational force needed to turn the tip of the bullet around the center of offset AOA caused by lift and unballanced air pressure across the bullet body, in fact, the need for rotational stability is inherently diminished as the shape of the ogive becomes less elongated -- I believe there is more than just bearing surface?

Anway, MV is a factor... ask anyone why the old Greenhill constant of 150 is not really appropriate today.

Reduced MV through faster ROT is evident in chrono work, while maybe not that significant, it is there -- a simple matter of the potential of stored energy, unless someone has found a way to gain energy? Again, the difference is not big, but it is measurable....

Throat errosion, well you hit it on the head I think. After testing during adoption of the 1:7 barrel, it was noted in the reports the "1/7 rifling promoted faster throat errosion" -- there were a couple of theories but the one that seems the most logical is the one that you present:

As you know, when the firing pin strikes the primer, the bullet does not move in one continuous and accelerating movement down the barrel... it actually comes to a stop twice (or even three times), we all know the first stop, the second is the one you reference -- When the bullet in pushed further into the throat, the rifling starts to etch the jacket and the bullet stops... at this point, the gases are jetting around the bullet and heat/pressures are at their highest. The increased angle of the faster ROT has a higher mechanical advantage to stall the bullet and this pause is longer on faster ROT... ergo the throat is subject to longer periods of high temp gas jetting. Granted, this is the simple description, but I am sure you get the jist of it?

One of the things that was noted when we tested GT rifling was the fact that throat errosion is noticably reduced, due to the above I belive -- however, there seemed to be no other advantage to GT, above and beyond what we got from three-groove, asymetrical cuts.

Also, it is not just arfcom that has the market cornered on people not knowing what they are talking about... but I would have to agree that reality is seldom a cornerstone of many thoughts on here.

Merry Christmas!

The 75gr and 77gr loads are much more effective than the lighter 55-69gr otterings, particularly with 16" and shorter barrels.   These heavy bullets need 1:7 rifling to stabilize properly.  The 1:7 rifling will also shoot the 55gr/62gr loads with little/if any accuracy degradation.  Bottom line is that I chose the 1/7 to optimize my weapon for the most effective ammunition available, in this case it's the 75-77gr loads (SAAMI or NATO spec).  Other than being able to buy a cheaper rifle(copy vs Colt), I'm not sure why anyone would choose a 1/9 vs 1/7.  

I dont think that throat erossion is a major issue in a colt 1/7, chromeliined barrel.   Even it everntually eroded after many-thousand rounds, just replace it.  Why compromise your ability to use the most effective loads?  By the way, I get 3100fps with Winchester M855 and 2850fps with Hornady NAto 75gr TAP, so I don't think my 1/7 barrel is hurting my velocity too much.

Quoted: The 75gr and 77gr loads are much more effective than the lighter 55-69gr otterings, particularly with 16" and shorter barrels.   These heavy bullets need 1:7 rifling to stabilize properly.  The 1:7 rifling will also shoot the 55gr/62gr loads with little/if any accuracy degradation.  Bottom line is that I chose the 1/7 to optimize my weapon for the most effective ammunition available, in this case it's the 75-77gr loads (SAAMI or NATO spec).  Other than being able to buy a cheaper rifle(copy vs Colt), I'm not sure why anyone would choose a 1/9 vs 1/7.   I dont think that throat erossion is a major issue in a colt 1/7, chromeliined barrel.   Even it everntually eroded after many-thousand rounds, just replace it.  Why compromise your ability to use the most effective loads?  By the way, I get 3100fps with Winchester M855 and 2850fps with Hornady NAto 75gr TAP, so I don't think my 1/7 barrel is hurting my velocity too much.

well I guess we know what brand of rifle you shoot and what your holiday beverage of choice is... Be honest dude, I have enough money saved to buy a Colt, just don't want one. Reading this board for months has left me feeling pretty convinced that Colt are rifles are great, but probably not THAT much better than the competition. In the end I decided to go with RRA because I couldn't  justify spending the extra money on a Colt, just can't believe they really are worth the extra $450 I would be spending. Maybe I'll learn the hard way.......

As for barrel twist, thanks for humoring a noob one more time guys. I will probably call Bushie and see if they can special order a 1/7 for a similar price. If they can I may get it, as being able to shoot the heavier stuff would be a nice option. If it is much more expensive than I will get the 1/9 for now. I will still be able to shoot most loads, and frankly I'm sure a 69 grain defense round would do a pretty impressive amount of damage if I ever was faced with a SHTF situation. Oh...and I don't want the 14.5" barrel because anyone tole me to, I just think the 16" looks kinds queer

Thanks folks, always helpful. Merry Christmas

Quoted: Throat errosion, well you hit it on the head I think. After testing during adoption of the 1:7 barrel, it was noted in the reports the "1/7 rifling promoted faster throat errosion" -- there were a couple of theories but the one that seems the most logical is the one that you present: As you know, when the firing pin strikes the primer, the bullet does not move in one continuous and accelerating movement down the barrel... it actually comes to a stop twice (or even three times), we all know the first stop, the second is the one you reference -- When the bullet in pushed further into the throat, the rifling starts to etch the jacket and the bullet stops... at this point, the gases are jetting around the bullet and heat/pressures are at their highest. The increased angle of the faster ROT has a higher mechanical advantage to stall the bullet and this pause is longer on faster ROT... ergo the throat is subject to longer periods of high temp gas jetting. Granted, this is the simple description, but I am sure you get the jist of it? One of the things that was noted when we tested GT rifling was the fact that throat errosion is noticably reduced, due to the above I belive -- however, there seemed to be no other advantage to GT, above and beyond what we got from three-groove, asymetrical cuts. Also, it is not just arfcom that has the market cornered on people not knowing what they are talking about... but I would have to agree that reality is seldom a cornerstone of many thoughts on here. Merry Christmas!

Okay, so how much longer will a 1/9 barrel last than a 1/7?  Lets assume both are chrome lined, 4150 barrels used on semi-auto guns.

Given that you've collected all this testing data can you give a percentage number of how much longer a 1/9 barrel will last?  How about a round count, how many shots 5.56 or .223 SAAMI spec ammo did it take to wear out the 1/7 barrels throat as compared to the 1/9?

I'd interested to how much the actual difference will be.

MTBF for the M4 is 9600 rds...

Granted, the difference between the barrel life of a 1:7 and 1:9 will be minimal, but that is not to say that is does not exist -- based on the situation you gave (average shooter, CL 4150 barrel) the information I have would indicate that he/she could expect a barrel life of 10-12K, the faster rot barrel would be on the low end and the slower on the high end. There is no indication that the difference between the two would be more than 1000 round however, or 6 to 9 percent.

So, for the average shooter does it matter? Probably not, I doubt that most people go through more than two or three barrels a year? say 25K on average? So, at the 25K per year, it would take a long time to realize the *free barrel* that came from going the long haul.

I am not arguing however that there is no place for 1:7, please look at my original post where I ask why go over 1.5 on stability factor? Not everyone shoots from a mat or a bench and a stability factor above 1.5 are is needed for some people (like I said, if you need a gun, get what you need)... I am only stating that the 1:7 barrel has a purpose that the original poster will never need and that for the same money he can go with a 1:8 SS and have what will most likely be the next "look at me" fad on arfcom anyway, and it will do everything that he wants it to do... I know, practical has no place on arfcom. Then again, he is a case of "get what you want"... from his own reply this gun is for posing "I just think the 16" looks kinds queer".

Oh, I am also pointing out that while maybe not that big of a deal... the facts support that rot does effect MV and errosion rate, but that is not really a secret...

It is more stable with a suppressor and less likely to cause baffle strikes in a worn out barrel.  Thats reason enough.  If you are concerned with throat errosion I wonder why you use 5.56 pressure ammo... that accelerates it too.

Quoted: It is more stable with a suppressor and less likely to cause baffle strikes in a worn out barrel.  Thats reason enough.  If you are concerned with throat errosion I wonder why you use 5.56 pressure ammo... that accelerates it too.

Sounds like a "no shitter"... but rotational stability would hardly have time to effect a bullet withing the confines of a can -- this is like the campfire story that used to go around about the "buzz cut" bullets that had little saw like teeth on the front and were supposed to cut "like a hole saw" through a target... people believed that too.

Deal is that the length of travel through a suppresor is say six inches or so, that means that with a 1:7 rot, the bullet is not even able to make a whopping full turn before it leave the can... rotational stability takes more than a half-foot to take effect. If you are getting baffle strikes, check your alignment, and a barrel should have gauged out LONG before it is this loose.

Oh, 5.56 is the standard of the US military... that's why

Quoted: MTBF for the M4 is 9600 rds... Granted, the difference between the barrel life of a 1:7 and 1:9 will be minimal, but that is not to say that is does not exist -- based on the situation you gave (average shooter, CL 4150 barrel) the information I have would indicate that he/she could expect a barrel life of 10-12K, the faster rot barrel would be on the low end and the slower on the high end. There is no indication that the difference between the two would be more than 1000 round however, or 6 to 9 percent.

That's about what I figured.  As far as I'm concerned that kind of difference is too small to matter 99% of the users.  Especially when you the variables like the type of shooting the user will be doing.

I've personally switched over to 1/7 because I like to be 100% sure the longer, heavier bullets will be shooting in the most ideal platform and I've found no accuracy or other issues shooting 45gr and 55gr out of my 1/7 as compared to my 1/9 barrels.  

The 1/7 twist basically meets and exceeds all the requirements I have for the 4150/CMV, 5.56 chambered, chrome lined general purpose barrels that I prefer to build carbines with.

BTW, I'm in no way saying that everyone should get the 1/7 because that's something you need to decide for yourself depending on how you will use your carbine or rifle.  

Actually, I don't care what twist you use as long as I can get what I want...

Ok guys, looks like I can get a 14.5" Bushie, with perm. attached Phantom and a 1/7 twist for about $245. So that is what I'm going to do. Sounds like if I want something that will do most everything then it may be the better choice. Thx.

Gunzilla covered the actual topic of discussion here, everyone else
kinda glossed over it.

mco119, save your money and get a 1/9.

I got a 1:7 because the development of the 5.56 is going heavy.  I figure it's better to have it and not need it than need it and not have it.

Quoted: Gunzilla covered the actual topic of discussion here, everyone else kinda glossed over it. mco119, save your money and get a 1/9.

thing is man, I need the 14.5" with perm Phantom either way, that is what I am paying more for. Until now I had been under the impression that the 1/7 barrels were more expensive, but the dealer I talked to today has both 1/9 and 1/7 Bushie for the same price. This surpirsed me, but if they are the same price, and the 1/7 handles heavier and lighter loads, why not  just get it and be done with it? Gunzilla added a lot of good info, but it doesn't seem that, price being equal, there is any big downside to the 1/7. And, again, maybe it will give me just a tad more flexibility. For now I am buying this gun solely for range work, but next year I graduate with a criminal justice degree and may enter police work. So there is at least a possibility that I may make this a patrol weapon, and therefore a possibility that later on I may care about being able to shoot heavier defense loads.

mco119,

Deal is, you asked about why the 1/7.  Most answers revolved around
performance.  You stated you cared about looks when you replied
about 16" looking ghey.  Yet, you want a 14.5" with a flash hider
to make it 16". Why not just go with the 16" barrel?  It will
give much better performance and look the same as a 14.5 with
flash hider.  Or you could just say to heck with what it looks like.

I can legally put whatever length barrel I want on my MG as it is a
RR.  Soooo, I go longer instead of shorter.  It makes the 5.56
perform more like it was intended.  You state it is for range work anyway.
Isn't the 16" (or greater) gonna be better than 14.5".
Also, if what it looks like is important then what difference does
it make if it is 1/7 and 1/9.

For me. I think 16" is a sweet spot for the 5.56.  And don't tell
anyone but I also like the dissipator.  And of course I also
have a 20" barrel.  But, right now, I got no upper shorter
than 16".  

I will never need to swing my M16 around in close
quarters so I get nothing with a shorter barrel.  Except the
looks of course.


YMMV............................

Quoted: mco119, Deal is, you asked about why the 1/7.  Most answers revolved around performance.  You stated you cared about looks when you replied about 16" looking ghey.  Yet, you want a 14.5" with a flash hider to make it 16". Why not just go with the 16" barrel?  It will give much better performance and look the same as a 14.5 with flash hider.  Or you could just say to heck with what it looks like. I can legally put whatever length barrel I want on my MG as it is a RR.  Soooo, I go longer instead of shorter.  It makes the 5.56 perform more like it was intended.  You state it is for range work anyway. Isn't the 16" (or greater) gonna be better than 14.5". Also, if what it looks like is important then what difference does it make if it is 1/7 and 1/9. For me. I think 16" is a sweet spot for the 5.56.  And don't tell anyone but I also like the dissipator.  And of course I also have a 20" barrel.  But, right now, I got no upper shorter than 16".   I will never need to swing my M16 around in close quarters so I get nothing with a shorter barrel.  Except the looks of course. YMMV............................

It is possible to care about the look of a weapon and about how it functions at the same time. For example, I have a DSA SA-58 FAL carbine-looks frickin sweet and is a heck of a performer. Versital in so may ways....in fact I imagine I will always prefer it to an AR in many ways. Why is it expected that I don't care about function simply because I also care about the apperance and mainly authenticity of a 14.5" barrel? I do care for function too, that is why the twist is of concern to me. When I take this to the range I will likely use Wolf or something similar. If this becomes a patrol rifle then carrying highly effective defense loads will be important. You've got some good points dude and I know where you are coming from.....but sometimes people on this site seem to think that you either have to choose a rifle with function as the only priority, or you choose a rifle becuase you want a pretty little safe queen. Looking cool and tearing crap up can go hand in hand

Merry Christmas again guys

You'll need 1/7" [or at least 1/8] if you want to shoot 70+ grain ammo [like the new 77 grain Mk262 stuff].

Also, I consider 16" mid-lengths to be a lot better than 14.5"ers... that's just me.

Quoted: Quoted: but sometimes people on this site seem to think that you either have to choose a rifle with function as the only priority, or you choose a rifle becuase you want a pretty little safe queen. Looking cool and tearing crap up can go hand in hand Interesting outlook. BTW - you're losing more performance with the shorter barrel, than you are gaining from the faster twist. But hey, as long as you look cool.

you are probably right man. However, I'm sure there are plenty of people of this forum who like the 14.5" too. In fact, last poll I saw the 14.5" was the most favored. Anyway, thanks for all the help guys. I am not an expert like some of you and it does me no good to get into a peeing contest over whether me wanting a 14.5" is dumb. Maybe it is, maybe it isn't. And if this ADCO....hey man I talked to you guys the other day, YOU are the folks installing it for me. Sorry you don't approve of my choice. Goodnight and Merry Christmas

Quoted: Quoted: Quoted: but sometimes people on this site seem to think that you either have to choose a rifle with function as the only priority, or you choose a rifle becuase you want a pretty little safe queen. Looking cool and tearing crap up can go hand in hand Interesting outlook. BTW - you're losing more performance with the shorter barrel, than you are gaining from the faster twist. But hey, as long as you look cool. you are probably right man. However, I'm sure there are plenty of people of this forum who like the 14.5" too. In fact, last poll I saw the 14.5" was the most favored. Anyway, thanks for all the help guys. I am not an expert like some of you and it does me no good to get into a peeing contest over whether me wanting a 14.5" is dumb. Maybe it is, maybe it isn't. And if this ADCO....hey man I talked to you guys the other day, YOU are the folks installing it for me. Sorry you don't approve of my choice. Goodnight and Merry Christmas

oh and I'm buying the RRA from you too.

Quoted: Quoted: Quoted: Quoted: but sometimes people on this site seem to think that you either have to choose a rifle with function as the only priority, or you choose a rifle becuase you want a pretty little safe queen. Looking cool and tearing crap up can go hand in hand Interesting outlook. BTW - you're losing more performance with the shorter barrel, than you are gaining from the faster twist. But hey, as long as you look cool. you are probably right man. However, I'm sure there are plenty of people of this forum who like the 14.5" too. In fact, last poll I saw the 14.5" was the most favored. Anyway, thanks for all the help guys. I am not an expert like some of you and it does me no good to get into a peeing contest over whether me wanting a 14.5" is dumb. Maybe it is, maybe it isn't. And if this ADCO....hey man I talked to you guys the other day, YOU are the folks installing it for me. Sorry you don't approve of my choice. Goodnight and Merry Christmas Hey, I try to talk many customers out of giving me their money, but no one has ever come back and said "why didnt you tell me"....

And I'm just giving you crap man. I work with an AK builder over on the other side of ar15.com and he has tried to set me straight a few times too, I respect the honesty. I think a big part of the problem here has been that I've confused everyone as to my priorities. I've asked questions about function and made comments about looks too, mentioned I want it for the range and also said I might consider it as a patrol rifle. I suppose my top priority for now is just to get the gun I want. I am, now anyway, more of a collector...at the minimum any law enforcement use will be a year away. At that time, if I have been convinced that a 14.5" barrel really is a bad choice, I may replace it with a 16" barrel. Maybe worrying about a 1/7 or 1/9 is silly for now.....and I don't think I would buy a 1/7 if it was 100 bucks more, but for the same as the 1/9 why not?

I found this to be an interesting read.   Discusses many of the issues in the prior posts.

http://elementconcepts.com/reference/Infantry-Rifle-Carbine1.pdf

SOSNBA,

Great link, thanks!

And of course, I am pleased that it supports my "hunch" about
16" being the best "compromise".

Quoted: I found this to be an interesting read.   Discusses many of the issues in the prior posts. http://elementconcepts.com/reference/Infantry-Rifle-Carbine1.pdf

That is a good read and it was also written by a member here (KevinB).

Quoted: I found this to be an interesting read.   Discusses many of the issues in the prior posts. http://elementconcepts.com/reference/Infantry-Rifle-Carbine1.pdf

The only use I have for a barrel under 16" is a dedicated CQB shorty... anything that I would carry around as carbine or patrol rifle will be at least 16" (but what the heck do I know)... Oh the Enhanced Battle Receiver has a 16" barrel... some of the .mil guys actually wanted a little bit longer -- but as you can see from Kevins doc, and other testing, the 77gr rnd performs outstanding out of a 16" and so much better than a 14.5 that it makes having the 14.5 a waste, really.

truth is that as much as we debate this stuff over and over, I wouldn't want to be hit by a 77 grain round from either a 14.5" or 16" barrel. Now the round from the 16" might be somewhat more effective, but I have this suspicion that I would not be a happy camper after getting hit from either bullet. And some of the reports I've read about the 77 grain rounds being used in Iraq have been pretty positive, and those are fired from standard 14.5" M-4 barrels. But I am no expert.....