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# my method

#22
12-17-2008, 02:28 PM
 Gold Member Join Date: Mar 2008 Location: N. Central Indiana Posts: 581
Re: my method

First off, I want to clarify that I'm not taking any "side", I am just posting this to hopefully learn something.

I see a reference to RPM, and bullet impact speeds in the testing. From my way of thinking, if the guns that are used to test the bullets have the same rate of twist, and the impact speeds are the same, then the RPM should be the same as well, correct?

Here's the way that I look at it.
Speed won't change the rotation of the bullet if the bullet is fired from barrels with the same rate of twist. (lets take gain twist barrels out of this equasion) 1 revolution in 10" is 1:10 no matter how fast the bullet is thrown. Yes, the RPM will change, but it only changes if the impact distance changes as well. (I'm also discounting any "skidding" of the bullet in the bore.)

I realize that the rotation of the bullet will have some effect on how the bullet expands, but has anybody been able to test and document just how much of an effect it actually does have?

Or should the term RPM be changed to RPD (revolutions per distance) when talking about bullet spin?
#23
12-17-2008, 02:42 PM
 Platinum Member Join Date: Jul 2002 Posts: 2,855
Re: my method

"eddybo" , you got a calculated BC of .910" for the 265gr HAT's ?

What is the BC of a 300 gr SMK??

I have some friends that shoot 338 Lapua and are intrested in the higher BC bullets.

I'm intrested in some of the 180gr HATs to try at long range out of a 308 , has anybody tested these yet to get an accurate BC
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#24
12-17-2008, 04:06 PM
 Platinum Member Join Date: Jan 2007 Location: MS Posts: 1,652
Re: my method

James, I can only say what works to plug into exbal to match my drops at the given velocities and distances, .71 works for the 180s and they are very accurate at 2850fps. My drop chart with it out to 800 is about as close as I have ever seen.
With the 265s I would say that with the information fed into exbal it is very close out to 1400 yards. I cant say that that is the correct number it is just the number that I was able to plug into exbal to get everything to work given the information that I knew. I feel confident enough to take a long poke at an animal using that number. I may have some other setting askew but I am almost positive that is the number I am using. I got a new phone so my old phone is now my dedicated shooting PDA I will look when I get home to make sure.

Quote:
 Originally Posted by James Jones "eddybo" , you got a calculated BC of .910" for the 265gr HAT's ? What is the BC of a 300 gr SMK?? I have some friends that shoot 338 Lapua and are intrested in the higher BC bullets. I'm intrested in some of the 180gr HATs to try at long range out of a 308 , has anybody tested these yet to get an accurate BC
__________________
I admit that I know just enough to be dangerous.....but dangerous at ever extending distances.
#25
12-17-2008, 04:55 PM
 Lightvarmint Posts: n/a
Re: my method

Quote:
 Originally Posted by mikebob eddybo you are right and i appologize to you and Mr. Henson for adding his name on my post and looking forward to your results. lightvarmit I did not see the pictures, but thank you for posting them. I will search for them right know.
One is in the thread about the harvest of the elk and the other is in the thread of the reference to the 300 SMK.

Lightvarmint
#26
12-17-2008, 05:29 PM
 Lightvarmint Posts: n/a
Re: my method

Quote:
 Originally Posted by esshup First off, I want to clarify that I'm not taking any "side", I am just posting this to hopefully learn something. I see a reference to RPM, and bullet impact speeds in the testing. From my way of thinking, if the guns that are used to test the bullets have the same rate of twist, and the impact speeds are the same, then the RPM should be the same as well, correct? Here's the way that I look at it. Speed won't change the rotation of the bullet if the bullet is fired from barrels with the same rate of twist. (lets take gain twist barrels out of this equasion) 1 revolution in 10" is 1:10 no matter how fast the bullet is thrown. Yes, the RPM will change, but it only changes if the impact distance changes as well. (I'm also discounting any "skidding" of the bullet in the bore.) I realize that the rotation of the bullet will have some effect on how the bullet expands, but has anybody been able to test and document just how much of an effect it actually does have? Or should the term RPM be changed to RPD (revolutions per distance) when talking about bullet spin?
Hello again,

At 3000 fps, the bullet rpm out of a 1-8" twist is 270,000 rpm and at 3000 fps the bullet rpm out of a 1-10" twist is 216,000 rpm.

At 2500 fps, the bullet rpm out of a 1-8" twist is 225,000 rpm and at 2500 fps the bullet rpm out of a 1-10" twist is 180,000 rpm.

In reality and in some cases, you can over come lack of twist with more speed. But, you are on or closer to the ragged edge. All the bullet needs is enough spin to create a stable environment. During the bullet flight, the velocity decrease is not in the same proportion as the decay of the rpm. RPM decays at a much lesser rate than the what one sees as the bullet slows to a halt. And, at the point where the bullet's flight is overcome by gravity and actually halts in flight over land and touches the earth, it still has gyroscopic spin. If this were not the case, we would not be able to shoot longer distances without fin stabilization.

When the bullet starts to deform and expand, it acts very much like a drill bit. It actually cuts a hole and if it deforms enough, it can spin portions of the jacket throughout the material that caused it to deform. Some folks even use metal detectors to locate the pieces of schrapnel. Rotational spin actually aids in creating the wound channel.

Finally, the actual bullet rotational spin is what causes the bullet to be stable. Sort of like a top that you spin on the floor. The faster it spins the more stable the top. Basically, bullets follow this same principle.

Anyway, some folks don't realize the speed at which these projectiles we shoot actually rotate. Even more interesting is that they get accelerated over a short distance...... Just think of the torque that they see when traveling down the barrel.

Lightvarmint
#27
12-17-2008, 05:42 PM
 Platinum Member Join Date: Jan 2007 Location: MS Posts: 1,652
Re: my method

Man if all your posts were like that I would like you. Very respectful and informative quotation of a viable theory. Are you on your meds today
In all sincerity good post.

Quote:
 Originally Posted by Lightvarmint Hello again, At 3000 fps, the bullet rpm out of a 1-8" twist is 270,000 rpm and at 3000 fps the bullet rpm out of a 1-10" twist is 216,000 rpm. At 2500 fps, the bullet rpm out of a 1-8" twist is 225,000 rpm and at 2500 fps the bullet rpm out of a 1-10" twist is 180,000 rpm. In reality and in some cases, you can over come lack of twist with more speed. But, you are on or closer to the ragged edge. All the bullet needs is enough spin to create a stable environment. During the bullet flight, the velocity decrease is not in the same proportion as the decay of the rpm. RPM decays at a much lesser rate than the what one sees as the bullet slows to a halt. And, at the point where the bullet's flight is overcome by gravity and actually halts in flight over land and touches the earth, it still has gyroscopic spin. If this were not the case, we would not be able to shoot longer distances without fin stabilization. When the bullet starts to deform and expand, it acts very much like a drill bit. It actually cuts a hole and if it deforms enough, it can spin portions of the jacket throughout the material that caused it to deform. Some folks even use metal detectors to locate the pieces of schrapnel. Rotational spin actually aids in creating the wound channel. Finally, the actual bullet rotational spin is what causes the bullet to be stable. Sort of like a top that you spin on the floor. The faster it spins the more stable the top. Basically, bullets follow this same principle. Anyway, some folks don't realize the speed at which these projectiles we shoot actually rotate. Even more interesting is that they get accelerated over a short distance...... Just think of the torque that they see when traveling down the barrel. Lightvarmint
__________________
I admit that I know just enough to be dangerous.....but dangerous at ever extending distances.
#28
12-17-2008, 08:24 PM
 Gold Member Join Date: Mar 2008 Location: N. Central Indiana Posts: 581
Re: my method

LV:

Thanks. I see clearly now! Well, clearer anyway. So RPM IS a valid measurement (i.e. helping us figure out how to stabilize a bullet).

Is there any rule of thumb on figuring out what rpm it takes to stabilize a certain bullet?

I know what you are saying about the bullet torque. In shooting a 110g .25cal Accubond @ 3400 fps in my 10 1/2# Vanguard, the rifle will torque the left bipod leg up off the shooting bench a good 2". It doesn't have a brake on it. My .257 Weatherby that does have a brake doesn't do that, but the bullet is 130g and has less MV (3250 fps). That one is a MarkV and weighs less than the Vanguard. I'm unsure whether that is a function of the brake or just the bullet/MV. (on the MarkV) Both guns are 1:10 twist

Last edited by esshup; 12-17-2008 at 08:27 PM. Reason: twist rate

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