Twist rate vs. altitude

[1shot]

I recently confirmed something I've been reading for some time. I'm shooting a 284 Shehane with 183 SMK bullets out of a 1-9 twist barrel. I'm in Ohio at about 1,190 ASL. The rifle would shoot, ok some days, other days but it would produce errant flyers with no explanation.
I recently traveled to Colorado to shoot a couple of matches with my long time friend at the Byers range at approximately 5,500 ASL.
Much to my delight, the rifle turned into a laser and posted a very good score. (A PR for me)
The bottom line is, the difference in elevation between the two locations proved to be a major factor in the performance of the rifle, so use enough twist for your bullet at your location.
I hope this helps,

Lloyd

 

[T-shooter]

Why do you think it is the barrel twist? I would think the much thinner air density would be the major factor. What distance are you shooting at when you have variations from one day to the next?

 

[hrlincoln]

Why do you think it is the barrel twist? I would think the much thinner air density would be the major factor. What distance are you shooting at when you have variations from one day to the next?

Dan Lilja has some good info on this and related topics...

http://riflebarrels.com/bullets-and-ballistics/

 

[spclark]

Why do you think it is the barrel twist? I would think the much thinner air density would be the major factor. What distance are you shooting at when you have variations from one day to the next?

It's the combination that's significant: higher altitude (thinner air) makes slower twist closer to 'optimum' for a particular bullet at efficient velocities. Reverse is true as you get closer to sea level and / or shoot in colder conditions.

Each 'condition' (twist, altitude, ambient temperature, bullet BC, velocity) influences how the others affect one another. No one single factor is any less important than any of the others when considered as a system, yet some - twist, velocity, temperature - may have more of an effect than the others as you approach the fringes of your conditional envelope.

 

[T-shooter]

Dan Lilja has some good info on this and related topics...

http://riflebarrels.com/bullets-and-ballistics/

Thanks, a good article.

 

[cowboybart]

I'll concur that elevation changes things. In a 6.5 a 9" twist will work for 140gr bullets where I live. At sea level sometimes it won't work. I haven't read to link yet, but it may not be the elevation, but other factors. Today the Relative Humidity is 12% here at 5100' in WY. My guess is the rel humidity is somewhere around 75% in OH, or places lower in elev. The relative humidity might be the deciding factor on bullet stabilization when the twist is borderline. Generally as you go up in elev the humidity goes down. This general rule is during steady barometric pressure. If there is a storm coming in then usually the higher elev gets the humidity (snow or rain).

 

[spclark]

Humidity (water vapor) is less dense than air so humidity's not as significant a factor, ballistics-wise, as air density (barometric pressure) adjusted for altitude.

Higher humidity sure makes for uncomfortable shooting conditions (nobody I know likes shooting in the rain!) while low humidity can dry you out quick & lead to dehydration issues... and the body problems that can bring.

 

[1shot]

Some really good points in the previous posts. I neglected to say this rifle is my 1K rifle, and I'm a sling/prone shooter. The relative humidity in Ohio is in the 70% range. The day of the match in Colorado the humidity was 35% and 82 *.
Thinking back, I tested this combination in December and it shot lights out, which lends credence to the statement of warmer vs colder air.
Thanks,

Lloyd

 

[Milo 2.0]

http://www.bergerbullets.com/berger-bullets-twist-rate-recommendations-by-eric-stecker-bryan-litz/

Start changing parameters ^^^ in this calculator, it'll become apparent altitude and velocity the biggest contributing factors.
I guess what I'm saying is, get 50 fps more from your load and it may be perfect on your home turf.

 

[1shot]

I just ran that program and came up with 1.44. That is marginally stable. So, I just happen to have a brand new 8.25 twist laying here all ready chambered up so I'll give that a whirl and use the 9 twist with another bullet.
Thanks,

Lloyd

 

[FeMan]

The two largest factors relating to stability of a given bullet are air density (density altitude) and twist rate. The velocity you drive the bullet at matters less than most people think. While higher velocities do give higher rate of spin (i.e. in rpms) and thereby increase gyroscopic stability, the higher velocities also increase the forces acting to destabilize the bullet at the same time (drag force applied of the center of pressure of the bullet). The relationship is not 1:1 and there is a slight benefit to higher velocity, but it is pretty small. If you have an unstable bullet, shooting it faster is likely not the answer. Increase the twist rate or shoot in thinner air is where you should concentrate.

 

[spclark]

Increase the twist rate or shoot in thinner air is where you should concentrate.

Exactly, and why Berger changed their 'recommended twist' a few months ago to reflect this effect.

Most of us can't have much to say about the air densities we shoot in but for selecting a barrel's twist? Different situation entirely.

Still, the games you play in (BR vs. Long Range vs. ELR vs. PRS, etc.) will have their own 'rules of thumb' that are proven to work in the conditions under which those games are played.

Ask questions! None of us know everything but some know more than others & we all learn from the discussions.