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Twist rate and bullet stability at 900 yards

 
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  #1  
Old 10-24-2003, 06:56 PM
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Twist rate and bullet stability at 900 yards

I switched to A-max bullets this afternoon with my 7mm Dakota rifle and got some near-bughole groups as I was working up a load. Then when I had time before dark for just one group at 900 yards the vertical spread was quite disappointing compared to those groups earlier at 100 yards.

My barrel is a 9 twist and Hornady recommends an 8 for the 162 grain A-max bullet. If this bullet was grouping well at 100 yards is it still possible the poor vertical at 900 was the result of not enough twist?

PS: The bullet box label does say 8 twist but I just noticed that on Hornady's website they call for a 9 twist.

[ 10-24-2003: Message edited by: Len Backus ]
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Old 10-24-2003, 09:19 PM
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Re: Twist rate and bullet stability at 900 yards

Len,

I'm trying to do the same thing with my 7 Ultra. And yes the 162's do say 8. The dealer said that my 9 1/2 twist should stabilize them at the velocity the ultra will shoot them. I believe him for he is a national record holder in long range. Besides, vertical stringing, I've been told by high power shooters, is caused by extreme spread.

db
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Old 10-24-2003, 09:22 PM
dwm dwm is offline
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Re: Twist rate and bullet stability at 900 yards

I have a 7mm RUM w/28 inch PacNor Super Match with a 1 in 11 twist. It stabilizes 162 AMAX/SST, 160 Accubond, and 168 SMK with no problems at 100 yds. Have yet to produce tiny little groups with it, but I don't think lack of twist is the problem. I have not shot at 900 though so I can't say what happens out there.

It did fail to stabilize 168 Berger VLDs on my first try with minimum loads. They produced keyholed or oblong holes in the target implying that they were tumbling when they hit the paper. Have not increased the load to determine if they will stabilize or not.

Have been focusing on the 168 SMK.

Some interesting reading can be found in an earlier post started by DC:

"Topic: Decelleration rate of bullet rpms at 500,1000, 1500 and 2000 yards

1. If you have a bullet fired in an 8 twist barrel at 3300 FPS, the formula for "in barrel" rpms would be;

12 divided by 8 twist= 1.5--times the MV of 3300 fps=4950-- times 60 = 297,000 RPMs."

If you have a 9 twist barrel at 3300 FPS:

12 divided by 9 twist= 1.33--times the MV of 3300 fps=4400-- times 60 = 264,000 RPMs.

That's still a lot of RPM. (11% change)

Interesting that with an 11 twist:

12 divided by 11 twist= 1.09--times the MV of 3300 fps=3600-- times 60 = 216,000 RPMs.

Which as far as rotating masses go is still a lot of RPM. (27% change)

My guess is that the box says 8 twist because it takes an 8 twist barrel when the muzzle velocity is low, say ~2700 FPS, to stabilize the bullet. I think that the twist rates printed on the box are calculated for non-magnum cartridges, not calculated for super magnums, like the STW, RUM, or Dakota.

Just for fun:

12 divided by 8 twist= 1.5--times the MV of 2700 fps=4050-- times 60 = 243,000 RPMs.

Which is more than the 1 in 11 twist at 3300 FPS!

I have wondered if real fast twist barrels (say 8 twist) suffer faster throat erosion than slower twist barrels. Thought being that it takes much more force to overcome the rotational moment of inertia cause by spinning up a bullet in a fast twist barrel. That will cause the bullet to accelerate forward slower keeping the flame in the throat longer which erodes the throat.

Did the temperature drop a lot between the bughole groups and your last group? I know the temp here can change 20 degrees from noon until right before dark. (Trying to be constructive after all the rambling.)

Doug

[ 10-24-2003: Message edited by: dwm ]
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Old 10-24-2003, 09:31 PM
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Re: Twist rate and bullet stability at 900 yards

Len,

I would highly doubt that the verticle was a result of your twist rate. If you can chrony them and see what your ES is, that will tell you how much effect speed should have. Now add 100 other errors that cause V and you have the problem solved.

Keep in mind, 50fps at 900 yds represents 10" of verticle to ADD to your group at 900 yds. That's just one problem.
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Old 10-24-2003, 10:40 PM
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Re: Twist rate and bullet stability at 900 yards

Len,....The vertical you speak of can be coming from the projectiles ,they're are some bad batchs,i can get u the number if you want.The others are great after serious sorting.....Twists ,don't worry about the 8 twist bit ,we have had good accuracy in a 10 twist down as low as 2700fps,so you have no worries ,as a mater of fact the Australian 1K group record is held with the 162Amax in an 11 twist at only 3080fps.The most accurate bullet i have ever used in 7mm at a 100 shot .190 to .213 three times in a row ,but would not shoot worth a penny a 1000yds so i don't bother there anymore.Why do you use such a big gun to shoot at only that distance???
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Old 10-25-2003, 05:04 AM
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Re: Twist rate and bullet stability at 900 yards

Great contributions from all. Thanks.

dbhostler and 4mesh063:
My velocity spread is quite good, not over 20 for extreme spread.

DWM
Temps were stable.

Jeff:
I'd like to see the bad bullet lot numbers. These are about a year old. I'm getting new ones this morning. The 7 Dakota pushes the 162 grain bullet at around 3300 FPS with 79 grains of H1000. In my 17 pound rifle it doesn't feel like a "big" gun. The 7 Ultra would probably need 20 grains more powder to get the same velocity and recoil would be much greater.

As I said, I only had time before dark for one group at 900 yards. If my long range groups are bad this morning I'll try sorting.
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Old 10-25-2003, 09:41 PM
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Re: Twist rate and bullet stability at 900 yards

Next chapter:

Well, I wouldn't have believed it! The A-max bullets are vanishing before they hit the target in one out of 4 firings. I reduced the velocity from 3300 down to about 3100 and it's still happening. THis is in a 9 twist barrel, hardly over stabilized. Even went to the bullet store (one hour each way) to get a different lot. Same result.

No wonder it seemed like my groups were so tight, although some still were. One of them was an honest 1/4 inch with at least 4 and probably five bullet holes. Final proof was in setting a target closer at just 30 yards away. Result was a bit of a jagged hole and some dark gray smudge just under the bullet hole.
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