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# Bullet Spin and Performance

#8
12-21-2008, 11:00 AM
 Platinum Member Join Date: Jan 2004 Location: Blackfoot, Idaho Posts: 7,858
Re: Bullet Spin and Performance

I tend to agree with Bryan based on my personal expansion tests.

Testing at close range, with long range velocities, with several brands of bullets which opened properly and penetrated in a straight line distance into the media was as bullet perfromance was as expected.

Bullets that were of insufficient velocity and/or suspected improper design immediately yawed upon media entrance, seemed to maintain that orientation through out their travel, did not travel in a straight line, and passed much further into the media. I can only suppose that the initial wound cavity with these bullets was much smaller than that of the bullets that performed properly.

The next step in my expansion testing is to connect the RSI device to the media container to determine variations in "shock" or what ever that measurement reveals.

What I have proven to myself is that SMKs need at least, if not a little more, than the 1800 FPS published FPS when launched at a 900 yd velocity. Maybe if launched at say 2700 FPS w/the media at 1K, the resulting increased RPM would make a bit of a difference on the bullet's ability more properly perform terminally in my media?
#9
12-21-2008, 11:03 AM
 Platinum Member Join Date: Jun 2001 Location: Potomac River Posts: 5,048
Re: Bullet Spin and Performance

Bryan

I ran some calculation of the angular velocity of the jacket of a 308 and while that number is bigger it was only as a prelude to an other calculation. Would you check to see if my calculations are correct. It has been many years since I had to calculate anything in radians.

-----------------------------------------------------

Muzzle velocity = 3000 ft/ sec

Barrel 1-12 twist = 1 rev/ft = 1 radian

3000 R/s w

Linear

2pi x r = v

308 caliber r = 0.154 inches x 2 x 3.1416 x 3000/ sec / 12 inches = 241.9 fps

Let us look at a 308 bullet from a 1-12 twist barrel fired at 3000 fps.

It has a forward velocity of 3000 fps and an angular velocity of 3000 rev/ sec at the muzzle

Converting the angular velocity to a linear velocity of a particle in the outer jacket can be done because we know the radius of a 308 bullet is half the diameter. That translates to 241.9 feet per second.

If we spin the bullet with a 1-10 twist barrel then it has a linear velocity of 12/10 X 241.9 = 290 fps or a 20 percent increase.

If we take the original barrel and decrease the muzzle velocity by 10 percent then the angular velocity is decreased by 10 percent.

We can decrease the velocity by 20 percent and increase the twist by 20 percent and keep the angular velocity constant.

If we go to a larger caliber bullet and keep the same barrel and muzzle velocity then the angular velocity will be the same but the linear velocity is more because the increase in the radius. So the difference between a 308 bullet and a 338 bullet is a radius of 0.154 inches versus a radius of 0.169 inches or about a 10 percent increase in radius and so a 10 percent increase in linear velocity.

Nonetheless, no matter what twist nor what muzzle velocity nor what caliber, what I calculate is that the linear spin velocity is going to be about 10 percent of the forward velocity at the muzzle.

The only reason angular velocity is important is to calculate angular momentum.

----------------------------------------------------------------------
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The Smokin Fur Rifle Club
#10
12-21-2008, 12:06 PM
 Platinum Member Join Date: Jan 2003 Location: The rifle range, or archery range or behind the computer in Alaska Posts: 3,276
Re: Bullet Spin and Performance

Quote:
 Originally Posted by jwp475 How much cyntrifical force can the bullet have? What studies or proof shows a greater wound channel from over spinning a bullet?
Studies done by myself and the testimonies of other experianced shooters. Some in ballistic gelitain, others on game itself.

I challenge you to make some ballistic gelitain and fire a 190 SMK from a 12x barrel at a given velocity into it, then shoot a 190 SMK with a 9x barrel and see for yourself.
__________________
Limits are only perceived by what has previously been achieved. Push the limits and good things will happen.

Go the distance or go home

Follow through matters. SEE THE FIRE!!!

August 2013 dall sheep: Passed one legal ram. Yes I regret it....
August 2013 Antelope: SUCCESSFULL!
October2013 Bull elk: Had to burn the tag.
It's been a GREAT year!
#11
12-21-2008, 12:26 PM
 Platinum Member Join Date: Mar 2007 Location: Montana Posts: 1,325
Re: Bullet Spin and Performance

My understanding of the physics of twist on the bullet are not scientific. I have searched for information on this and then try to put it in practical terms that I can wrap my head around. So hear goes what I think I understand.

The stability factor of a bullet is dependant on the overall length of a bullet, the bearing surface of the bullet, the muzzle velocity of the bullet, and the twist of the barrel. What the bullet is made out of, and how the bullet is balanced will have some bearing on how much spin it will need, and or how much it can take.

What I understand about over-stabilization is that too much spin can bring out the imperfection of a given bullet and cause it to go down range in a circular around it's flight path. Also it can cause the jacket to separate from the core partially or completely break the bullet into pieces.

I understand that it takes less stability factor for shooting long range (beyond 500yrds) than for shooting less than 500yrds. This I believe has to do with the bullet nosing over and coming down with the point in line of the bullet path. If the stability factor is higher it will cause the point to stay in the upward position of the beginning of the flight and enter the target in an upward angle. I believe this can be very subtle and cause the bullet to not deform properly, and not stay on the intended line through the target.

In the point of hunting we need the bullet to stay on the path that we aimed the bullet through the intended target. A bullet that may tumble will tend to pick it's own path after impact. This could be very bad on a quartering shot as you can imagine. As I understand it the higher rotation of the bullet forces it to stay on the aimed path. So one can conclude that as much spin as the bullet can physically take would be good for short range hunting (under 500yrds).

In my quest for information on this subject I am beginning to think I need to have two loads for a given rifle depending on the distance of the shot. Perhaps this is hair splitting. I don't know. If the target is dangerous game it becomes more important for personal safety reasons.

I also have concluded that the further we push these bullets (velocity and range) the more noticeable these factors become. The more radical the bullet designs get, for increased bc's, the more attention needs to be paid to the twist.

This is a great thread, I hope we can get more involvement from the really smart people around here.

Thanks for the time guys, Steve
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To hunt... or not to hunt...? What a stupid question.
#12
12-21-2008, 12:35 PM
 Lightvarmint Posts: n/a
Re: Bullet Spin and Performance

Quote:
 Originally Posted by meichele Studies done by myself and the testimonies of other experianced shooters. Some in ballistic gelitain, others on game itself. I challenge you to make some ballistic gelitain and fire a 190 SMK from a 12x barrel at a given velocity into it, then shoot a 190 SMK with a 9x barrel and see for yourself.
Meichele

10-4.

And, if you cannot find some ballistic gelatin perform the following steps in order as an alternative:

1) Go to your local range and dig out your favorite bullet that is mushroomed or has exposed cutting teeth on the circumference.

2) Select your favorite power drill and chuck up the bullet and start it up at full speed. Bear in mind that it probably will only be 10% of the rated rpm of the bullet. The more rpm the more damage that will be done and the more rapidly it will be done.

3) Next, either just think about what would happen if you grabbed the spinning bullet with your bare fingers. Now go find a piece of ham or roast and drill a hole in it with nothing but rotational force. I would not recommend grabbing it with your fingers, but it will provide a small visual example of the damage a rotating bullet can impart on flesh and meat when it is only at 10% of actual rotational velocity............ Just imagine the potential damage at full rpm and full range velocity.

Finally, once you have done this, you will more fully understand the damage rotating mushroomed and/or fragmented bullets impart on targets and media. Now, you have to ask yourself will this be MORE or LESS significant damage than a broadhead launched at 300 fps from a comound bow at a range of 30-50 yards.....

Also, if the bullet is stable upon impact and not in a yaw condition, it would tend to travel in the original direction upon impact. And, if you want to know what happens at 1000 yards, then test at 1000 yards to get the most accurate results.........

Conducting long range testing at long range removes the doubt and forces us to discuss other topics.

Lightvarmint
#13
12-21-2008, 03:20 PM
 Platinum Member Join Date: Jan 2003 Location: The rifle range, or archery range or behind the computer in Alaska Posts: 3,276
Re: Bullet Spin and Performance

Quote:
 Originally Posted by RockyMtnMT My understanding of the physics of twist on the bullet are not scientific. I have searched for information on this and then try to put it in practical terms that I can wrap my head around. So hear goes what I think I understand. The stability factor of a bullet is dependant on the overall length of a bullet, the bearing surface of the bullet, the muzzle velocity of the bullet, and the twist of the barrel. What the bullet is made out of, and how the bullet is balanced will have some bearing on how much spin it will need, and or how much it can take. What I understand about over-stabilization is that too much spin can bring out the imperfection of a given bullet and cause it to go down range in a circular around it's flight path. Also it can cause the jacket to separate from the core partially or completely break the bullet into pieces. I understand that it takes less stability factor for shooting long range (beyond 500yrds) than for shooting less than 500yrds. This I believe has to do with the bullet nosing over and coming down with the point in line of the bullet path. If the stability factor is higher it will cause the point to stay in the upward position of the beginning of the flight and enter the target in an upward angle. I believe this can be very subtle and cause the bullet to not deform properly, and not stay on the intended line through the target. In the point of hunting we need the bullet to stay on the path that we aimed the bullet through the intended target. A bullet that may tumble will tend to pick it's own path after impact. This could be very bad on a quartering shot as you can imagine. As I understand it the higher rotation of the bullet forces it to stay on the aimed path. So one can conclude that as much spin as the bullet can physically take would be good for short range hunting (under 500yrds). In my quest for information on this subject I am beginning to think I need to have two loads for a given rifle depending on the distance of the shot. Perhaps this is hair splitting. I don't know. If the target is dangerous game it becomes more important for personal safety reasons. I also have concluded that the further we push these bullets (velocity and range) the more noticeable these factors become. The more radical the bullet designs get, for increased bc's, the more attention needs to be paid to the twist. This is a great thread, I hope we can get more involvement from the really smart people around here. Thanks for the time guys, Steve
That sums it up very very well. I dont always know how to translate my thoughts to a key board and you have done it to near perfection!

Thank you Steve!

Yes it may be hair splitting (having two loads for a given rifle depending on the distance) but that really would depend on how much of a perfectionist you are and what distances youre talking about. My opinion only here. I am a 1K shooter NOT a 1K+ shooter. I find that "the perfect" load works just fine from 0-1000 yards. More may be different.
__________________
Limits are only perceived by what has previously been achieved. Push the limits and good things will happen.

Go the distance or go home

Follow through matters. SEE THE FIRE!!!

August 2013 dall sheep: Passed one legal ram. Yes I regret it....
August 2013 Antelope: SUCCESSFULL!
October2013 Bull elk: Had to burn the tag.
It's been a GREAT year!
#14
12-21-2008, 03:27 PM
 Silver Member Join Date: Mar 2006 Location: Nebraska Posts: 178
Re: Bullet Spin and Performance

This all good info. and I really appreciate it. How much would a bullet spin in an animal if you were using a 1:10 twist? I can see where spinning in a drill would illustrate some spin but if a bullet only turns maybe twice while in an animal this wouldn't be an accurate representation of what it does to an animal, would it? I envision this as a cork screw from hell instead of a spinning mushroom of death.

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