barrel length/stablization

[skizzums]

I have been shopping for a couple weeks for a 22-24" bull barrel for .223. I want to use this for 600yard highpower and want to stay with my 75-80gr ble bullets. Why is it almost all the 24" barrels only come in a 1:9 twist(excluding a few high end or out of stock optiona)? Question is, would the longer barrel be able to stablize better even with slower twist rates? I would be okay with even a 1:8, but I am accustomed to, and prefer a 1:7. Does it not matter with 8 more inches of barrell? I am just a little confused as I thought the market for 24" barrels would be geared toward long range shooters.

This is for ar15 btw.

Thanks

[Jimro]

1:9 at 24" should stabilize most 75gr bthp bullets adequately for 600 yard high power shooting.

A 1:8 twist will stabilize 77gr SMKs out of a 20" barrel. Haven't tried 80gr Amaxes yet.

I wouldn't turn down a 1:9 twist 24" barrel for High Power, although if I had the choice I would pick a 1:8 barrel.

I have four High Power service rifle uppers, 2 with 1:8 twists, one with 1:7 and one with 1:7.7 twist. They all shoot 75gr BTHP loads nicely, even with 20" service rifle barrels.

I do know of one competitor who earned his High Master with a Bushmaster XM15E2 rifle with 20" 1:9 twist barrel, shooting 69gr SMKs across the course.

I hope this is helpful.

Jimro

[T. O'Heir]

It's because of the newish heavy bullets. However, barrel makers make barrels with calibre standard twists. If you look at a regular .243 calibre hunting rifle you'll find that even 'varmint' barrels are rifles for deer weight bullets, not varmint bullets. Typical .223 barrels had 1 in 12 until the heavy bullets came along.
Eight more inches of barrel will give you slightly higher velocities, but it's the twist that does the stabilizing, not the length.

[skizzums]

I want the short answer, all things being equal.....will a longer barrel stabilize better than a shorter one?
I would assume not, but that's my question.

[Bart B.]

Yes, but maybe not enough.

Increasing a given twist .223 barrel length 5" makes bullets leave about 150 fps faster (on average) which spins bullets about 4% faster. But this is only valid testing the longer one first then cutting off 5 inched at its front. A 5" shorter barrel may shoot bullets faster than another that much longer. All barrels of a given length for a given cartridgr don't shoot a given load he same velocity.

Bullets need a small range of rpm's to stabilize them. Any combination of muzzle velocity and twist that's good will work great.

Spin in rpm = muzzle velocity in fps X 720 / twist in inches.

[phudd]

I want the short answer, all things being equal.....will a longer barrel stabilize better than a shorter one?
I would assume not, but that's my question.

The short answer is not. It is more a factor of efficiency of the propellant burn which yields a more consistent velocity. Shorter barrels give lower velocity and do not buck the wind as well. A ridiculously long barrel would actually reduce accuracy as any length beyond the point where the powder is burned slows the bullet by friction.

http://www.thetruthaboutguns.com/201...-and-accuracy/

[JD0x0]

length, in this case, is only significant due to velocity.

Will the extra velocity make up for a slower twist? 9/10 I'd say the answer is 'no'
The increase in velocity usually doesn't compensate for a slower twist.

Take the barnes 62 grain TTSX in a 1:9
Berger stability calculator says an SG of 1.17 @ 2900FPS with a 1:9'' twist.
With a 1:8 twist at the same velocity SG = 1.49

If you could somehow propel that bullet to 5000FPS in the 1:9 twist the stability factor is still only 1.41
You need to push that bullet to 5900FPS in a 1:9 twist to match the stability of that bullet in a 1:8 twist.

More velocity tends to help stability, but it's not really going to make up for a twist rate that is too slow, in most cases.

[skizzums]

Okay. Thanks all, i will continue the hunt for a 1:7 or 8 24" bbl

[Jimro]

A 75gr Hornady BTHP has a stability factor greater than 1.3 out of a 9 twist barrel, even as slow as 2440 fps.

But if you want to launch 80gr bullets, you probably need a faster twist, as at best you'll score a "marginal" level of stability with the longer bullets (about a 1.15 with an 80gr Amax).

Fulton Armory says that 24" Criterion 1:8 heavy barrels are in stock. http://www.fulton-armory.com/barrelf...ainmuzzle.aspx

White Oak also has an option in stock: http://www.whiteoakarmament.com/xcar...76&cat=&page=1

I don't think you could go wrong with either a Criterion or Wilson barrel.

Hope this helps.

Jimro

[skizzums]

Thanks Jim. I like the fitted gas block on the white oak, just one thing to make life easier

[Unclenick]

A couple of minor corrections. The 77 grain SMK and Nosler CC bullets are actually stubbier and shorter than any of the Hornady 75's, so they need less twist to stabilize them and not more. Bullet length is the dominant consideration in stability because greater length puts the center of pressure further forward of center of gravity in pointed bullet shapes. That gives the overturning drag force a longer lever arm to work against bullet stability with. The 77 grain SMK are stabilized by a 9" twist in an ICAO standard atmosphere, but there's not much margin and the stability factor is sub-optimal. Warmer, less dense air would be better.

I don't know what a ble bullet is. Did you perhaps mean a VLD (very low drag) or a frangible bullet? If so, you have extra length relative to weight, and that requires a still faster twist for the reasons I gave above. If you specify the bullet make and weight, we can be more helpful.

[Jimro]

Unclenick,

I think you meant that the Hornady Amax was longer than the 77gr SMK, not necessarily the regular Hornady 75gr BTHP.

The following data was pulled from JBM's length listings:

Length listed for Hornady 75gr BTHP 0.980"

Length listed for 77gr SMK 0.994"

Length listed for 77gr Nosler CC 0.980"

Length listed for Hornady 75gr Amax 1.105 The length of the plastic tip for the projectile is listed as 0.141" for other Amax or Vmax bullets.

I only listed the 1:8 barrel stabilizing 77gr SMKs because I shot Mk262 through an AMU M16A4 with a 1:8 barrel a few years back, so I know that load combination works well, I didn't mean to imply that a 1:9 wouldn't stabilize a 77gr SMK just that I haven't done it.

Jimro

[Unclenick]

Right you are on the A-max. For some reason my eyes didn't move far enough down the page. I'm probably prejudiced by the fact the A-max is the only Hornady 75 I have. It looks from the data that their 0.980" 75 grain match bullet and the 0.994" 77 grain bullet, both at 2800 fps, have about the same stability factor. Very slight edge to that 75.

On another forum different Australian shooters reported both good shooting and problems with the 77's from a 9" twist. I'm not sure of the conditions, though. As I said, it's marginal and your 8" twist is better.

One thing that's funny about the charts with more twist listed for greater bullet weight, is they don't tell you they are assuming same bullet specific gravity and shape type. Only under those limited conditions does weight have a positive correlation to length. In reality, if you can increase a bullet's weight without changing its shape, as by using a more dense alloy, then you need less twist at any given velocity, as the gyroscopic stiffness is greater for the greater spinning mass. Conversely, if two bullets are the same weight have the same ogive and tail shape, but one is made from a less dense alloy, like solid copper, then the less dense bullet is longer and needs faster twist to stabilize it, despite having the same weight.

[skizzums]

Well, I don't have a specific brand of bullet that I want to only use. I want a barrel that can handle all of the heavyweight .223 bullets. I would prefer to stay with a 1:7, but just can't seem to find many that don't have a 1:9 twist and was just wondering why that is. I would figure most people who slap a 24" bbl on an AR are wanting to do long range shooting and would want to utilize the highest bc bullets they can find. I just didn't know if the extra length would help with stabilizing and maybe the faster twist wouldn't be as necessary. I usually use hornadays 75 or smk 77s

[Bart B.]

Longer bullets for a given caliber typically need faster twists when they're leaving at the same muzzle velocity. However, if the barrel's length is increased to give 15% more velocity with heavier bullets, one doesn't need to go from a 1:8 twist to a 1:7 one to make up the difference needed to spin the heavier one faster to balance it.

[Jimro]

skizzums,

There is a lot going for the 1:8 twist. A 1:9 twist should stabilize a base 75 BTHP and 77 SMK, but it is just over the calculated base, which is why some shooters report excellent results and some have to step back down to the 68~69gr bullet weight range.

Honestly the 1:7 twist was adopted only to stabilize the long tracer rounds, and is too much twist for anything but the 85 gr and heavier bullets. Of course you can't get enough powder in a 223 case to make good use of the 85 and heavier bullets, so most shooters stick with the 75 to 82gr range.

If you drop the coin on a Kreiger barrel, you can get a 1:7.8 twist, which I can't tell the difference in accuracy from a 1:8 twist.

That being said, I wish I'd kept that 1:9 twist Bushmaster now for a 100 yard reduced course only rifle. At 100 yards even cheap milsurp ammo shoots good enough to be competitive if you are with a service rifle.

Jimro