The Down Range Forum
Member Section => Tactical Rifle & Carbine => Topic started by: blackwolfe on February 21, 2009, 12:27:53 PM
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Ok, so I managed to get an AR before the election and the EBR and PIF frenzy started. Have another that was put away that was built during the ban and is lacking some evil features. Don't know much about them and have been asking a few questions here and getting good answers. Although barrels seem to be hard to get, I have also been gathering some parts and tools to change the barrel on one and to build another, but I have a few questions on twist rates and bullet weights.
What bullet weights work best in the following twist rates and barrel lenghts?
16" barrel
1:7
1:8
1:9
20" barrel
1:7
1:8
1:9
Thank again for your help
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If I remember right ( feel free too correct me you guys) the lighter the bullet the tighter the twist.
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Actually a tighter twist can stabilize a much heavier bullet. Such as a 1:7 is great for an 80grain bullet. 1:9 can handle the mid 50 grain bullets up to the 70 grain bullets. 1:8 is for the 60 grain to upper 70 grain bullets. Those numbers are just based on my experience.
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So will a lighter bullet stabilize in a tighter twist barrel, such as a 45-55 grain bullet shooting well in a 1:8 twist barrel. I have heard people use the term that a bullet can "over stabilize." I always figured a bullet was either stable or not stable.
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I don't know that you can "over stabilize," but if you put too much spin on a light bullet it will destabilize or even fly apart. That might be what they were refering to when they said "over stabilize."
There is more to stabilization than just twist. It is a combination of twist, bullet weight, and bullet speed. That is where handloading gets interesting ::)
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In the AR-15 the most common twist rate is 1 in 9". This will stabilize up to 69 grains quite well. I've read that some get fairly good accuracy with up to 75 grains, but that's pushing it. for 70 grains and above a 1 in 7" twist is preferable, and even necessary. You can try some 75 grain ammo in a 1 in 9" twist and see how it performs. The worst is they will keyhole, but you'll only be out a box of ammo. For very light bullets from 45 to 55 grains a 1 in 12" twist will work. Many .22-250 barrels are rifled with a 1in12", or a 1 in 10" twist. The longer the bullet the more twist is required to stabilize it. Weight is a second consideration to length. Bill T.
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I just read an article on this. It's a (slight) misconception to say that bullet weight determines proper twist for bullet stabilization. It's actually bullet length. However, since heavier bullets are generally longer in the same caliber, you can make that statement and be basically right.
In general
1:7 will stabilize 55-77gr bullets
1:9 will stabilize 45-62gr bullets
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Getting closer. you need to look at the driving band of the bullet(the portion of bullet that comes into
contact with the barrel). Heavier bullets being longer naturally have a longer driving band. I shot 75gr bthp
in my .223 for years with absolutely superb results. 55gr fmj have very little driving band. There are
variations depending on manufacturer. I shoot frangible now and have almost the same driving band in
a 42gr bullet that I had in the 75gr bullet.
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The twist rate is all about bullet RPM in the end, that's where barrel length and velocity with the same twist rate vary. Here is a pretty good article on the subject along with the formula for figuring it out.
http://accurateshooter.wordpress.com/2008/06/03/calculating-bullet-rpm-spin-rates-and-stability/
On another note, most jacketed rifle bullets do not have driving bands, Lapua, Norma and Barnes are exceptions, as they definitely do have driving bands in some offerings.
I have a custom Krieger barrel, 26" 1/6.5" twist for using 80-90 grain VLD bullets in one of my AR's, it shoots the 77 grainers well too. The 77 grain is the longest bullet that will still feed and fit in GI magazine.
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Good info guys, I'm learning a lot in an area I am not as familiar with as I'd like to be.
Here is another link to a calculator for barrel twist. It was on the page m25 referenced above, but this is the straight link to it.
http://kwk.us/twist.html
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In a rifle, the entire bullet is typically covered in copper or a similarly soft alloy, so the entire bullet is its
own driving band. Driving band does not mean that there is specifically a band of lead or copper wider
than the bullet as in the case of artillary shells or muzzelloaders. It is merely the portion of the projectile
that comes into contact with the barrel.
So, unless you have a magic bullet(Arlen Spincter) that flies down the barrel without touching it, then
you must have a driving band.
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Barnes bullets have "driving bands" because of their solid copper construction. They have no soft lead core to upset. The Hornady solids are similar in nature. Bill T.
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1911 junkie has wikepedia on his side, but I knew what driving bands were long before the internet was running down some bodies mamma's leg. The wiki version does indicate the term from military artillery shells having a specific band of softer material for the mating of shell to bore. Then go's on to say as 1911 said, " a rifle bullet is it's own driving band " then what do you call a bullet with grooves cut concentric to the bullet, with gaps in between?? Of course driving bands, not band. We have always used the term, for bullets without grooves, " surface area " A bullet without grooves has more surface area, in contact with the bore, than one with grooves in a similiar bullet design, ergo less friction, if they are both the same diameter, even less friction if the bullet has lubrication in those grooves.
http://tbn2.google.com/images?q=tbn:xHROfmuQNAjUkM:http://members.aol.com/SCOLL63101/public/GS1
http://www.lima-wiederladetechnik.de/7,62-mm/Bilder/7,62-mm-DJG-1.jpg
http://www.hurog.com/images/silver/fired_bullets.jpg
http://dutchman.rebooty.com/images/bullet2.jpg
http://www.gsgroup.co.za/magnumhv.jpg
Notice in the photo that they have concentric rings, called driving bands, on the shank and only these match the groove diameter of the barrel. This means that the lands engage (‘bite’ into) far less copper than with conventional bullets. HVs thus build up pressure slower and in general, achieve higher velocities than conventional bullets of similar weight in any given calibre. Gerard claims that the reduced bearing surface is not the only reason why his bullets are faster, but as the explanation of this is very technical I suggest that you contact him for further details. The above information is basically all the average reloader needs to know.
The point is, driving bands are exactly that or We would not have a definition, separate bands that reduce friction with or without lubrication. They make a huge difference in cast lead bullets. Bearing surface on the other hand also makes a huge difference, 9mm bullets come to mind 1st, hollow point will always have more bearing surface over a solid, in a given bullet weight and usually more accurate, the VLD bullets have much more Bearing surface + Higher ballistic coefficient = more potential for accuracy.
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Most centerfire rifle bullets don't have driving bands. Despite what it says in Wikipedia the whole bullet isn't a driving band. It has a bearing surface but that isn't a driving band. On the bullets that do have driving bands, they're wider than the rest of the bullet. The main bearing surface of the bullet rides in the bore while the driving bands are forced into the rifling. If you look at an artillery shell it's obvious why they need driving bands. The rest of the shell is steel or cast iron that won't engage the rifling. The driving band is copper or some other material that's softer than the steel of the barrel. The driving band is the only part of the shell that the rifling engraves.
BTW, Golden Saber pistol bullets have a driving band at the heel. That's the only pistol bullet I know of that's made that way. Since the jacket is brass instead of gilding metal it's harder, so the rest of the bullet is bore diameter and only the driving band fits the rifling.
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Now, now. I think somebody needs to to go take a blood pressure pill, maybe a nice relaxing haircut.
We have always used the term, for bullets without grooves, " surface area "
Why didn't you just say that in the first place?
If it will keep you from having a stroke, I'll call it "surface area".
How did the internet run down my mamma's leg? Al Gore hadn't even invented it yet.
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1911, I said somebodies leg, your not old enough, for it to have been your momma, if your under 40, and I hope you are, please disregard the comment.
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O.K. I feel better now. ;)
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In the 1/9 twist barrels I have had good luck up to 75 grains if the barrel is a 20". My 16" barrels generally won't do it, although I have one that will. The 69gr bullets are meant to duplicate the 62gr penetrator round (M855) in length. As stated, the bullets above 77/78 grains won't be mag length.
I have one with a 1/12 barrel, and it prefers the 45 to 55 grain bullets. :) I just had to have one retro...
-Bidah