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Temp sensitive powder and fps ?

 
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  #1  
Old 12-08-2013, 11:13 PM
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Temp sensitive powder and fps ?

I have a accurate load of IMR 4064 I worked up for my 204 and chronoed a month ago at 3800 at around 55 degrees F at about 3500 ft. I did not confirm drop so I am not certain how accurate the fps is. Also, the scope I am using is a leupold Vx 3 with newly installed m1 turrets.
Today was super cold at 9 F and around 2500 ft. My zero was set at 100. I went to 500 and shot a solution based on 3800. As expected it was low. Solution called for 6.1 moa. Wind was being mean and 7 to 8 then 3 to 4. Ugh. Well first round was low, 7". Next two confirmed 6 to 7" low. Adjusted to 7.1 moa but went high 4". Running out of light and ammo I adjusted to 6.75 and sent 5 rounds. I pulled 1 and the rest seemed fine. See attached picture.
So now that I am thawed out and running the numbers it looks like 3600 is the correct speed to give the right solution at 500. Is it possible to see 200 fps variance in a 45 degree temp swing?
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  #2  
Old 12-09-2013, 12:25 AM
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Re: Temp sensitive powder and fps ?

Quote:
Originally Posted by BrentM View Post
So now that I am thawed out and running the numbers it looks like 3600 is the correct speed to give the right solution at 500. Is it possible to see 200 fps variance in a 45 degree temp swing?

That comes out to around 4.4 fps per degree of temp drop. Thats not impossible, but you'd be better off double checking. I really don't know anything about that powder.

Put some ammo inside your clothes close to your body for a good while so your powder temp can get close to 90 and do the same test to see where the group impacts.

That will give you both a broader temp swing, and a second confirmed drop at a known range...which seems to be the variable you're missing to know for sure if the data is reliable.
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Old 12-09-2013, 06:33 AM
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Re: Temp sensitive powder and fps ?

Happened to find that IMR 4064 is insensitive to temperature changes thanks to the Hodgdon extreme tour:

http://www.hodgdon.com/smokeless/extreme/page2.php#top

Go down to the third graph and see how 4064 fares against varget. Sure varget is better but a 46 fps swing from zero to 125 gr seems pretty good to me!

Perhaps there are other factors causing your comeup to be off.
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Old 12-09-2013, 08:55 AM
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Re: Temp sensitive powder and fps ?

Yes. Could be. Original fps was not verified. It records my 6.5 fine but this 20 cal pill might be hard for the chrono to read. I did shoot 3 different powders and compare for speed. Picked most accurate and fastest.

Doubtful the scope has anything to do with it. Thought it was strange to this big of a diff.
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Old 12-09-2013, 11:30 AM
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Re: Temp sensitive powder and fps ?

I agree verification is needed. One thing I question about advertisments etc such as that is the conditions in which the test was conducted. At 27.5 grains of powder and a 40 grain pill the velocity might very well be affected at a greater level then say 55 grains and a 140 grain pill. I am not versed enough to know any of this though so it is an interesting experiment for me.

I will test the tracking of my scope and shoot again soon at different distances to confirm drop data. If the scope was off that much it would be off by 15%. That seems like a lot to me. This scope is almost new and Leupold is not known for bad quality so I would hope the click values are in fact close to .25" per click. I am just now getting this combo set up and addressing any potential issues. Looking forward to getting it dialed in, fun to shoot.
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Old 12-09-2013, 01:36 PM
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Re: Temp sensitive powder and fps ?

I went back through this and noticed I didn't turn coriolus on. I was shooting due west and should have come up another .1 for CE.

Also, in my excitement to go shoot I forgot to set my scope center height. In shooter it was set at 2.0 from bore center. If it was off one way or the other by .25" it will show a difference of .20 at 500. I have a 50mm objective, 20MOA rail, and high rings. It could be between 1.75 and 2.25.

Oops. Rookie!
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  #7  
Old 12-10-2013, 02:52 AM
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Re: Temp sensitive powder and fps ?

Fast and easy scope height measurement:

Measure diameter of scope and divide by two. This is product A.

Measure diameter of bolt body and divide by two. This is product B.

Measure the space between the bottom of the scope and the top of the bolt installed in the action. This is product C.

A + B + C = scope center to bore center. Can be done to 0.001". Most ballistic programs accept only to 0.01"
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