Twist rate and bullet stability at 900 yards

[Len Backus]

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 ]

 

[dbhostler]

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

 

[dwm]

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 ]

 

[4mesh063]

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.

 

[a.JR]

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???

 

[Len Backus]

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.

 

[Len Backus]

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.

 

[Brent]

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.

Len, would you expand on what you mean by that a bit more?

Interesting...

 

[Len Backus]

Brent

The hole looked normal going in but not clean and round on the backside of the target backer. This material is a piece of that honey-comb stuff that normally takes bullets very cleanly.

 

[Darryl Cassel]

len

The jackets on the Hornady bullets are very thin.

We tried them in our 7/300 Weatherbys but found that the 168 Gr Sierra MK worked MUCH better and gave better groups in the 9 to 9 1/4 twist barrels.

We ran them at 3250 FPS with great success on animals in a 30" barrel.
Later
DC

 

[dbhostler]

Len,

I've been running the same bullet at 3280fps and haven't had a problem. I moly coat these A-Max bullets, but that shouldn't make a difference. 96grn Retumbo lit by a 9 1/2 M. Interesting though...

db

 

[Len Backus]

Darryl

I will never, ever, have your wind reading skills so I am trying to work with higher BC bullets.

.625 vs somewhere near .500 for the SMK.

 

[Guest]

[ 10-26-2003: Message edited by: Crispin Goodall ]

 

[Brent]

Len,

The reason I asked, and I was wondering if you were thinking they were unstable at close range (30yds) and that's why they might be exploding down range, but this is how I understand it, correct me if I'm wrong tho:

When the bullet leaves the muzzle it can be upset by the blast, and also MV and spin is high, but as the bullet heads down range, velocity decay is much greater than spin rate decay, so now bullet spin is able to better dampen yaw. I'm really not sure the bullet becomes "more" stable down range as a result, but it seems if slight key holing is present at close range, even 100-200 yards, that does not necessarily mean the bullet will not stabilize completely soon after this and never go unstable until it's near subsonic speed or even after that.

I know many people that don't recommend shooting groups any closer than 200 yards for this reason only.

It's not a 7mm, but close, but dad's 6.5WSM has pushed the 140 Amax over 3250 and they've never came apart in his 9 twist, sounds strange they come apart down a lot lower too. Maybe Hornady could sort out the problem for you, maybe they'd like to even have some bullets back to analyze???

I'm fixin to move up and try to hit 3500-3600 fps with the 178 Amax in the 30-338 LI so I'm wondering if they'll even make it at that speed now. All 9 hit the paper at 3400 fps or so at 300 yds the first time out with them.