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twist rate

 
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  #1  
Old 01-17-2002, 05:03 PM
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Join Date: Jul 2001
Location: Walla Walla, WA
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twist rate

While I have a good load worked up for my 300 Win Mag with the 220 Sierra Matchking I am considering a lighter yet high bc bullet that will give flatter trajectory. As mentioned in my previous post I can only rezero up to 550 yards and still be within the hash marks on my scope. I called Barnes and they told me that their 30 caliber 200 grain X bullet is 1.476" long and that it only needs a 1:10 twist to stabilize (presumably even at 30-’06 velocities). Warren Jensen has told me that a 1:10 is fine for his 180 grain J36 bullet. I do not know how long it is but I imagine it is more than 1.4". I have some of Gerard Schultz's 160 HV bullets on order. I would have ordered the 173 HV but it is 1.496" long and he said that I would need a 1:9 twist. I would really rather use the 173 than the 160 because even though it starts out slower it ends up surpassing the 160 due to its much higher bc (650 versus 515).

For example, starting the 160 at 3400 and with a zero of 275 yards:
At 700 yds velocity = 2180 fps; energy=1690; drop=65” and drift=27”

The 173 at a muzzle velocity of 3270 would have the following figures at 700:
Velocity=2300; energy=2040; drop=65”; and drift=22”

So clearly the 173 outperforms the 160. Now for the big question- does one really need a 1:9 twist for the 173 HV, especially in light of the fact that the 220 Sierra Matchking (one guy I have talked to is shooting the 240 Sierra Matchking from a 1:10 twist 300 Win Mag without any instability), the 200 Barnes X and the 180 J36 will all stabilize just fine from a 1:10 twist?

Any thoughts? Thanks, Rufous.
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  #2  
Old 01-17-2002, 07:21 PM
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Join Date: May 2001
Location: Arco, ID 83213
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Re: twist rate

Rufous,

The length and weight of a bullet are not the determining factors in gyroscopic stability. It is the relationship between the center of reverse air pressure under the ogive and the center of mass. The actual equation is fairly complex. The greater the distance between these two the faster the twist(spin) needed to keep it pointed nose forward.

If Gerard recommends a 1:9 for best performance then I would believe him. He's knows more about his bullets than anyone else. If you don't think he's correct, about the only way for you to find out is to purchase some and shoot them. Sometimes things don't always work out the way the math says it should. That is usually because there are factors in the equation that were not properly accounted for.

Warren
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  #3  
Old 01-17-2002, 10:50 PM
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Join Date: Jul 2001
Location: Walla Walla, WA
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Re: twist rate

Warren, thanks for the info. As I remember it, Gerard did say that his 30 cal 173HV would probably shoot accurately enough in a 1:10 but that it may tumble when it is penetrating through the animal. Obviously it would be simple enough to shoot the 173 grain bullets and see if they group well in my rifle. The thing I am so nervous about is the question of whether or not they would tumble once connecting with the animal, thereby possibly not penetrating enough. I do not know what to think. I would love to try his 173 HV (honestly I would love to try your 180 J36 but do not think I can swing the cost sorry to say. I do not doubt that they are worth the cost but I am on a tight budget). Clearly I want to put the animal down as quickly as possible. There are so many options and very few (if any) are perfect. It seems no matter what choice is made there is a compromise. I am not an ultralong range hunter (I think 500 yards is a very long shot on big game under the limitations I have of hunting on foot with a 9.5# rifle). The scope I plan to use does not have a target turret that can be used to rezero at ranges beyond 550 yards for the 220 Sierra or 650 yards for a bullet such as the 160 or 173 HV or your J36 bullets. So maybe it would be best to just shoot the 220 Sierra and figure if the animal is beyond 550 I need to get closer or pass it up. Trying to figure out the way to go, Rufous.
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  #4  
Old 01-18-2002, 04:40 PM
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Location: Arco, ID 83213
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Re: twist rate

Rufous,

You can test to see if they will tumble in media. Use wet newspaper,phone books, or catalogues. Don't stack them neat. You need at least 40" depth of material. In the first 2" of material place a 2" green pine board with a 30 degree cant angle. This will simulate a bone, but the wood has to be uncured and green. Mark the front of the material so that you will impact at the correct place and angle to stay in the media. Don't forget to allow for the scope offset above the barrel centerline.

If it maintains a straight line through this concoction then you do not have to worry about it's performance in game. It will do, OK.

Warren
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  #5  
Old 01-18-2002, 07:58 PM
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Location: Walla Walla, WA
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Re: twist rate

Warren, thanks for the tip. Why is it necessary to cant the board? Bones are not canted at a 30 degree angle. If I cannot easily find an uncured green pine board could I just try this in wet paper? Would the bullet be more likely to tumble going through a board (or bone in the real world example) than just through wet paper (or hide and flesh)? I have never shot a bullet into wet paper. How would I tell if the bullet had tumbled? If it is sideways or backwards when I find it then it would be obvious that it had tumbled but what other clues might I look for?

I do not understand your last sentence. Are you saying that you are quite confident that I will find that the 173 HV will not tumble and will in fact be quite stable in a 1:10 twist? Thanks again, Rufous.
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  #6  
Old 01-18-2002, 08:00 PM
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Location: Walla Walla, WA
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Re: twist rate

Warren, another question- at what range should I do this? Obviously I would not want the bullet to tumble at any range but is it safe to assume that if it does not tumble at muzzle velocity then it will not tumble at slower velocities? Thanks, Rufous.
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  #7  
Old 01-19-2002, 07:07 AM
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Posts: 2,369
Re: twist rate

"The length and weight of a bullet are not the determining factors in gyroscopic stability. It is the relationship between the center of reverse air pressure under the ogive and the center of mass. The actual equation is fairly complex. The greater the distance between these two the faster the twist(spin) needed to keep it pointed nose forward. "


Warren, I love it when you talk like that!!

In my mind's eye I always equated the twist rate required as a function of length as I imagined trying to spin a squatty toy top in comparison to a #2 Lead Pencil. The spin required for this is that required to overcome only gravity. I never gave too much thought to the pressure caused by the bullet flying through air. Good thing I'm not a rocket scientist!!

rufous

I think the board requires the cant because many bones a round(ed), not flat. It'd be more common to bit a bone off-center than directly at 90 degrees to the flight of the bullet.
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