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Bullet Stability in Winter Weather
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slimjim
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PostPosted: Sat Nov 23, 2013 10:01 pm    Post subject: Bullet Stability in Winter Weather Reply with quote

We have had winter storms since Thursday night with ice storms predicted for tomorrow and Monday. There was a hole in the weather this afternoon so I ran out to the range. It had warmed up to 35F and the wind was below 15 mph out of the North. I wanted to test my hunting bullets in colder temps for a possible January hunt. English Mike had sent me some Barnes 150gr TSXs to test. Barnes listed them as requiring a 1:9.5 twist. The on-line JMB stability calculator, which uses the Miller Equation, had their stability as acceptable in warmer weather but marginal at the colder temps.

www.jbmballistics.com/...ab-5.1.cgi

The stability was comparable to the 130gr GMXs I hunt with. I had previously contacted Hornady directly about the GMX stability and was told they use a more sophisticated calculator and the bullet was good to 10F in a 1:10 twist.



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Last edited by slimjim on Sat Nov 23, 2013 10:09 pm; edited 1 time in total
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slimjim
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PostPosted: Sat Nov 23, 2013 10:05 pm    Post subject: Re: Bullet Stability in Winter Weather Reply with quote

When I got to the range, I had to use my windshield scraper to clear a 1/2" of ice off the shooting bench. Below are stability charts of the .270 hunting bullets I shot today. The Nosler Accubond Long Range was purposely a reduced load as I'm preparing to do further testing with it and can reduce the yardage I shoot at to 500 yards and still get equivalent performance at 800+ yards. The stability calculations of all three bullets were comparable.

Both the GMX and AB LR were both stable. The TSX was not. Shows the impact of colder weather on bullet stability and how this calculator is only an estimate. Looks like a 1:9.5 twist is required for the 150 TSX or you need to shot it in warmer weather.



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chambered221
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PostPosted: Thu Nov 28, 2013 9:07 am    Post subject: Re: Bullet Stability in Winter Weather Reply with quote

Slim, you listed your velocity at 2750..... was that reading based upon shooting in cold weather ???

It's likely that your not achieving that velocity if the reading was taken in warmer weather. If that's the case your input data to the calculator would be flawed.

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slimjim
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PostPosted: Thu Nov 28, 2013 10:11 am    Post subject: Re: Bullet Stability in Winter Weather Reply with quote

chambered, the velocities listed were through my chrono on the day I shot. I typically find that my 130 GMX load can lose up to 40 fps in colder weather. Small velocity changes don't seem to impact bullet stability as much as air temperture/density.

When I'm hunting elk in NM where longer range shots are the norm, I carry my magazine with round loaded next to my body so they stay warm until I insert the magazine and chamber a round. That extra 40 fps makes a difference at 400 yards.

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slimjim
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PostPosted: Sat Dec 28, 2013 9:12 pm    Post subject: Re: Bullet Stability in Winter Weather Reply with quote

It warmed up to 62F today so thought I'd try the 150gr TSX, again. My latest load development had the velocity up to 2825 fps so I was hoping it would be stable in flight with the warmer air temperature. It was stable and shot 0.5 MOA at 250 yards. I was pleased so I did a terminal performance test. Even though the bullet was stable in-flight, the instant it touched the water jug, the bullet went unstable and yawed 180 degrees through the bullet trap. By instant, the bullet had tipped/yawed before it even penetrated the outside of the jug as indicated by the keyhole impact in the plastic. The bullet did no expand and looked like it could be loaded and fired again.

Looks like I'm going to have to wait for 90F to perform terminal performance testing.

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Last edited by slimjim on Sun Dec 29, 2013 11:50 am; edited 1 time in total
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Elvis
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PostPosted: Sat Dec 28, 2013 9:21 pm    Post subject: Re: Bullet Stability in Winter Weather Reply with quote

hmmm or does it preform well as a tumbling on impact type load???

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slimjim
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PostPosted: Sat Dec 28, 2013 9:29 pm    Post subject: Re: Bullet Stability in Winter Weather Reply with quote

Elvis, gelandangan likes a tumbling bullet. For a tumbler, this bullet stays at 90 degrees. Much better than the Berger VLDs that flip and penetrate tail first.

However, remember when it was colder, this bullet wasn't even stable in-flight. It needs a twist faster than 1:10 which is what Barnes recommends.

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Last edited by slimjim on Sun Dec 29, 2013 3:22 am; edited 1 time in total
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gelandangan
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PostPosted: Sat Dec 28, 2013 10:07 pm    Post subject: Re: Bullet Stability in Winter Weather Reply with quote

Well, I like tumbling boolits because it does open up the flesh much better at subsonic speed where it doesnt have much of the hydrostatic shocks.
At supersonics speed, it doesnt matter much, but boolits side on does usually have much larger surface area than it have mushrooming face on .

Usually at subsonic speeds, the tumbling is caused by instability due to loss of balance (the bullet do travel with lighter end in front) on impact.
At supersonic speed, the cause of tumbling are usually a lot different.

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slimjim
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PostPosted: Sun Dec 29, 2013 3:31 am    Post subject: Re: Bullet Stability in Winter Weather Reply with quote

gelandangan wrote:
Usually at subsonic speeds, the tumbling is caused by instability due to loss of balance (the bullet do travel with lighter end in front) on impact.

gelandangan, this makes sense. The Berger VLDs have a large hollow cavity under their tip so, when the bullet starts to yaw, the center of mass is very much aft on the bullet compared to the center of drag causing the bullet to be more stable traveling butt first.

Also, because the 150gr TSX is so large and stays sideways as it penetrates, I can tell it gives a big wallop to the bullet trap. Much more than the VLDs. Also, the center of mass and drag must be pretty close to each other because the bullet stays 90 degrees during penetration.

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