So I'm about a week away from purchasing my first AR. Before I do this though I'd like some help understanding the twist rates on barrels. Most of the guns I'm looking at start with a 1:9 twist with a few holding options for a 1:8 and one company giving a 1:7 option. I know that twist relates to how much spin it put on the round exiting the barrel and that somehow that translates to accuracy and distance, after that...it gets like my freshmen year of college...dark and hazy. Also, some companies talked of chrome lined barrels while others have different options, what gives there? Any help is appreciated, ya'll are the best!
bigpapi83
get a book by John Wootters entitled, The Complete Book of Practical Handloading. It discusses a lot of topics concerning matching a load to your gun, how twist rate and bullet sectional density work together, etc. I actually found it at the local library, of all places... I've forgotten most of it because I have not put itto practice, sorry I could not be of more help.
Sir, in practical terms, twist rate determines what bullet weights the barrel will stabilize. Generally, the faster the twist rate, the heavier the bullet it'll stabilize. In other words, if you run too heavy a bullet for your barrel's twist rate, you'll get poor accuracy, and the bullets may keyhole--go through the target sideways.
In a .223 AR, a 1:12 TR will stabilize up to about a 60-grain bullet; it's really designed for 55-grain bullets. 1:9 will stabilize up to 69-grain bullets, and some barrels will also handle 75- and 77-grain bullets. 1:8 will stabilize up to 80-grain bullets. Some 1:7 barrels will stabilize some 90-grain bullets, but not all. A lot of match guys who are using 90-grain bullets are running twist rates of 1:6.5.
Note that fast twist barrels usually still perform well with lighter bullets. For example, 55-grain bullets work just fine in a 1:7 barrel. You can run into trouble with flimsy varmint hunting bullets designed for slow twist rates when you run them in fast twist barrels, though. Because such bullets' jackets are very thin, the bullets can literally fly apart from the increased centrifugal force of a fast-twist barrel. It looks like a little puff of gray smoke downrange.
Chrome lining a rifle's bore increases it's lubricity and makes it easier to keep clean in crummy field conditions. Such bores also tend to last longer than plain steel bores, sometimes four times as long. Military M16 series rifles all have chrome-lined bores. Match shooters tend to avoid chrome-lined bores because the chrome can't be applied absolutely evenly, which results in tight and loose spots in the bore; for most purposes, this isn't a problem, but it's bad for absolute accuracy.
What you intend to do with the rifle determines what twist rate you want, as well as whether you want a chrome-lined bore. For a match rig to be shot from 200 to 600 yards, you want at least 1:8 twist rate and no chrome lining. For a match rig to be used at 1,000 yards, you might want a 1:7 or 1:6.5 and no chrome. For a plinker or "end of the world" anti-zombie rifle, you probably want 1:9 and chrome lining.
Hope this helps, and Semper Fi.
Ron H.
In a .223 AR, a 1:12 TR will stabilize up to about a 60-grain bullet; it's really designed for 55-grain bullets. 1:9 will stabilize up to 69-grain bullets, and some barrels will also handle 75- and 77-grain bullets. 1:8 will stabilize up to 80-grain bullets. Some 1:7 barrels will stabilize some 90-grain bullets, but not all. A lot of match guys who are using 90-grain bullets are running twist rates of 1:6.5.
Note that fast twist barrels usually still perform well with lighter bullets. For example, 55-grain bullets work just fine in a 1:7 barrel. You can run into trouble with flimsy varmint hunting bullets designed for slow twist rates when you run them in fast twist barrels, though. Because such bullets' jackets are very thin, the bullets can literally fly apart from the increased centrifugal force of a fast-twist barrel. It looks like a little puff of gray smoke downrange.
Chrome lining a rifle's bore increases it's lubricity and makes it easier to keep clean in crummy field conditions. Such bores also tend to last longer than plain steel bores, sometimes four times as long. Military M16 series rifles all have chrome-lined bores. Match shooters tend to avoid chrome-lined bores because the chrome can't be applied absolutely evenly, which results in tight and loose spots in the bore; for most purposes, this isn't a problem, but it's bad for absolute accuracy.
What you intend to do with the rifle determines what twist rate you want, as well as whether you want a chrome-lined bore. For a match rig to be shot from 200 to 600 yards, you want at least 1:8 twist rate and no chrome lining. For a match rig to be used at 1,000 yards, you might want a 1:7 or 1:6.5 and no chrome. For a plinker or "end of the world" anti-zombie rifle, you probably want 1:9 and chrome lining.
Hope this helps, and Semper Fi.
Ron H.
I agree with Ron but also realize that weight is commonly used to compare twist. Bullet length is a concern also. VLD bullets while they may weigh the same as others typicaly need a faster twist to stabilize properly.
Agree with what was posted. If I recall correctly the 1:7 twist was initially spec'd by the .mil for rifles to be able to stableize the longer tracer rounds, which were in the mid to high 60s in weight. I know that the Marine Corps has put alot of 55 grn and 62 grn rounds through 1:7 M4A1s ( very few in service. narrows down the guessing) with no problems, out to 500 yards for annual quals. I personally think 55 grain is the lowest I would go in a 1:7 twist, as I don't like spending money on ammo that don't hit the target. I have enough of those issues on my own without adding ammo issues.:dope:
I have heard that the 1:8s will stabilize almost everything that a 1:7 will. An acquaintence had a bushy barrel marked 1:9 that was gauged at 1:8, and handled the 77 gr. stuff just fine. If I am not mistaken many SPRs are barreled 1:8 SS and are tailored to shoot the 77 gr. Mk262.
pat
I have heard that the 1:8s will stabilize almost everything that a 1:7 will. An acquaintence had a bushy barrel marked 1:9 that was gauged at 1:8, and handled the 77 gr. stuff just fine. If I am not mistaken many SPRs are barreled 1:8 SS and are tailored to shoot the 77 gr. Mk262.
pat
Depends on what you will be shooting. You really should be fine with any of them. I just this month bought my first AR, after having been issued several. After much reading and studying I determined that the 1:7 milspec suited my needs the best. Since I intend to play in the 55-77 grn realm (my carbine is a work gun,) it seemed the best route, since I was going with either a Colt or LMT. Most of the 80+ gr bullets are specific to benchrest type of shooting, in fact I have seen disclaimers on ammo boxes of 80 gr. bullets that they are intended to be hand loaded into the breech, not magazine fed. I know the ammo guys at www.ar15.com played with 100 grain bullets a while back, and decided that they had a pretty stiff "rainbow" trajectory.
What do you want your gun to do? A 1:9 should realistically be all your needs are primarily a plinker first, defensive weapon second. Mine is the opposite. I intend to shoot a couple of thousand rounds a year in training, running my guns hard. I have no need for rounds lighter than 55 grains, and hopefully will be using 75 gr. TAP rounds for work, but the current spec is 55gr. softpoints. Plinking will be alot of fun, but training for using my gun against people is my primary consideration.
If your rifle interest is primarily professional, then go to www.10-8forums.com and pay attention to what Pat Rogers, Tim Lau, Jeff Chudwin, and others have to say. The learned over there vastly prefer 1:7 milspec for a be-all do-all rifle. Sully Sullivan is another poster over there, and he builds alot of 1:9s, so while there are learned folks over there, they are professionals, and can disagree on what is best.
If your rifle is more of a hobby for you, then a 1:9 midlength gas system will probably last as long as you own the rifle. www.ar15.com is a good hobbiest site, if you can deal with all the chest pounding, internet authorities who may or may not know what they are discussing, ect. I don't spend time there because there are literally thousands of new posts on a daily basis, and I don't want to keep up with them. That and the members there are probably not as knowlegeable as others. Many are "often wrong, but never uncertian", and internet pissing contests are common. Many knowlegeable posters are shouted down by those whose only experience is carrying (thre is a difference difference between carrying one and using one to actively kill people) a rifle in the military for x-years, or have seen/handled/talked to by people who have credible training and experience.
Another thing is to consider is the Barrel/Bolt are wear items. In a carbine gas system with a 1:7 expect about 6000-12000 to be the life of the barrel, depending on usage. Expect a mid length gas system with a carbine barrel and a 1:9 to last about 10000+ rounds. Every 1000 rounds I shoot a mag or two at 300-400 yards, and check the groups. When they start to decline and loose accuracy, then it is time to replace the barrel. If you only shoot out to 1-200, you can easily miss minor degradations in accuracy that can indicate that your barrel is shot out.
Like everything else your needs should drive what kind of equipment you get.
I really gotta stop posting on so little sleep.
pat
What do you want your gun to do? A 1:9 should realistically be all your needs are primarily a plinker first, defensive weapon second. Mine is the opposite. I intend to shoot a couple of thousand rounds a year in training, running my guns hard. I have no need for rounds lighter than 55 grains, and hopefully will be using 75 gr. TAP rounds for work, but the current spec is 55gr. softpoints. Plinking will be alot of fun, but training for using my gun against people is my primary consideration.
If your rifle interest is primarily professional, then go to www.10-8forums.com and pay attention to what Pat Rogers, Tim Lau, Jeff Chudwin, and others have to say. The learned over there vastly prefer 1:7 milspec for a be-all do-all rifle. Sully Sullivan is another poster over there, and he builds alot of 1:9s, so while there are learned folks over there, they are professionals, and can disagree on what is best.
If your rifle is more of a hobby for you, then a 1:9 midlength gas system will probably last as long as you own the rifle. www.ar15.com is a good hobbiest site, if you can deal with all the chest pounding, internet authorities who may or may not know what they are discussing, ect. I don't spend time there because there are literally thousands of new posts on a daily basis, and I don't want to keep up with them. That and the members there are probably not as knowlegeable as others. Many are "often wrong, but never uncertian", and internet pissing contests are common. Many knowlegeable posters are shouted down by those whose only experience is carrying (thre is a difference difference between carrying one and using one to actively kill people) a rifle in the military for x-years, or have seen/handled/talked to by people who have credible training and experience.
Another thing is to consider is the Barrel/Bolt are wear items. In a carbine gas system with a 1:7 expect about 6000-12000 to be the life of the barrel, depending on usage. Expect a mid length gas system with a carbine barrel and a 1:9 to last about 10000+ rounds. Every 1000 rounds I shoot a mag or two at 300-400 yards, and check the groups. When they start to decline and loose accuracy, then it is time to replace the barrel. If you only shoot out to 1-200, you can easily miss minor degradations in accuracy that can indicate that your barrel is shot out.
Like everything else your needs should drive what kind of equipment you get.
I really gotta stop posting on so little sleep.
pat
Twist rates are no more than a function of stabilizing a bullet.
The only thing that makes a bullet need a faster twist is length. Long bullets need a faster twist. Long does not always mean heavy.
Years back PRL made tungsten core match bullets. 80-100 gn bullets that would stabilize in a 9 twist. They were as long as the Sierra 69s.
As noted in above posts, heavies generally need a fast twist and lights will work in any twist.
I recommend a 8 twist so you can shoot anything from a 35 to an 80.
The only thing that makes a bullet need a faster twist is length. Long bullets need a faster twist. Long does not always mean heavy.
Years back PRL made tungsten core match bullets. 80-100 gn bullets that would stabilize in a 9 twist. They were as long as the Sierra 69s.
As noted in above posts, heavies generally need a fast twist and lights will work in any twist.
I recommend a 8 twist so you can shoot anything from a 35 to an 80.
Specific purpose?
This is generalization, but I hope I can clear some things up.
Heavier bullets are usually longer bullets. This varies somewhat due to bullet construction. Barrel twist is matched to bullet length, but weight is often an indicator of length.
For AR type rifles, 1:9 is a good all purpose twist. It will stabilize the common 55gr plinking stuff, down to 45gr HP's for woodchucks and up to the 68-70gr target stuff.
1:7 is great for the heavier bullets, 68-70gr up to the 85-+ match target loads. These are often impractical for anything but prone slowfire at service rifle matches due to magazine max length. The heavy loads are too long and have to be loaded singly through the ejection port.
If you plan on using it as a varmint or target rifle, the faster twist will let you get more range out of it (heavier/longer bullets retain more velocity), especially if you get into handloading. The over 65gr stuff isn't common in factory loads.
If it's a general use rifle, the odd woodchuck out to 2 or 300 yards and paper at the range, I'd opt for the 1:9. I have a 1:9 twist 24" upper that I use with 45gr Speer TNT's for woodchucks and surplus ball for plinking. I've shot woodchucks past 200 yards with it and gotten nickel sized groups at 100 with it.
This is generalization, but I hope I can clear some things up.
Heavier bullets are usually longer bullets. This varies somewhat due to bullet construction. Barrel twist is matched to bullet length, but weight is often an indicator of length.
For AR type rifles, 1:9 is a good all purpose twist. It will stabilize the common 55gr plinking stuff, down to 45gr HP's for woodchucks and up to the 68-70gr target stuff.
1:7 is great for the heavier bullets, 68-70gr up to the 85-+ match target loads. These are often impractical for anything but prone slowfire at service rifle matches due to magazine max length. The heavy loads are too long and have to be loaded singly through the ejection port.
If you plan on using it as a varmint or target rifle, the faster twist will let you get more range out of it (heavier/longer bullets retain more velocity), especially if you get into handloading. The over 65gr stuff isn't common in factory loads.
If it's a general use rifle, the odd woodchuck out to 2 or 300 yards and paper at the range, I'd opt for the 1:9. I have a 1:9 twist 24" upper that I use with 45gr Speer TNT's for woodchucks and surplus ball for plinking. I've shot woodchucks past 200 yards with it and gotten nickel sized groups at 100 with it.
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