Temp sensitive powder and fps ?

[BrentM]

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?

 

[c_bass16]

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.

 

[AZShooter]

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.

 

[BrentM]

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.

 

[BrentM]

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.

 

[BrentM]

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!

 

[Kennibear]

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"

 

[BrentM]

Thanks. That is nice and easy method. I measured it last night and 2.0 was pretty much dead on. I had just thrown in a number originally based on experience with such combos but had not confirmed it. Following the shiny object made me forget.

I think 3600 is probably close. I will shoot drop again when I go back out. The POI at 6.75 MOA is close to 2.75 low from POA. If I come up .5 to 7.25, based on measuring the target 3600 is getting me quite close. I can fine tune from there.

 

[Kennibear]

Do you chronograph your loads?If not you will be chasing your results 'till Kingdom Come. From your post I gather you are estimating your MV.

All recent production IMR powder is "Extreme" Technology. Doubt temp is an issue. Back when the NRA tested this they used 4895, 4350, and 4831 -surplus. The 4831-surplus was bad. But the 4895 wasn't too extreme, about 80fps as I recall. But this is not your problem.

You have to have an accurate (<1% preferred) Muzzle Velocity to start. So buy, beg, borrow a chronograph.

But from your results I would guess your BC figure is wrong, that it is not what the bullet maker claims (surprise, surprise!). I use Sierra GK and MK for most of my shooting and their BC's are very accurate. That is not true for every maker. Because High BC bullets sell better the #'s are inflated. When you side by side the bullets and two are pretty much identical then how is it they vary so much in BC?

Many posts to this website have commented on reducing the BC # to conform the data to the prediction of the Ballistic Programs. I bought a second chronograph to get down range velocity (on the same shot, each shot) to get an accurate BC. I believe this is your problem.

Hope others will chime in on this. Dialing in your data is what it is about. Lots of experience on this site doing just that.

KB

 

[Kennibear]

Re read you original post. Your chrono not reading is a problem. But you need to check the actual speed at 7F degrees. Hard to believe it is -200fps.

Sometimes shooting MUCH closer to the screens helps. My 45's read every shot. The 556 needs to be no more than 6" above the slits and dead center. I built armored sleds for my chronographs so I don't hammer them when I do this.

Huge density altitude change between your two range sessions. The cold one may have been at or below sea level DA. 55F @ 4500' altitude may be close to standard conditions. Dial in the changes in conditions and see how close it conforms.

Finding solutions to this sort of thing is the both frustrating and fascinating thing about this hobby. Kind of like women.....

KB

 

[BrentM]

I shot the 204 at 10ft off and between the 4 and 6" mark on the chrono. I don't mind doing it old school via drop confirmation either. I like to do both. BC of the bullet would have to be considerably off for it to make a 6.5" low impact.

For example if I left the 3800MV in the solution and tweaked BC it would be .230 vs the .275 or about 21% off. Now, looking at Berger 40, Nosler 40, and Sierra 39, I see the numbers all over the freaking place. Berger is the lowest of the group and Sierra the highest.

Any thoughts on the .275 BC of the vmax vs .225 of the Berger or .239 of the Nosler? Sierra is .287.

 

[Kennibear]

Go with the BC. But I would shoot over the chronograph every time if possible. I do and the numbers always mean something.

I'm guessing the BC listed is in fact BS. Try three different ranges w/ same zero. Check the BC change against that info.

A lot of shooting but someone has to do it!!!

KB

 

[BrentM]

I will dig the chrono out and shoot over it again as well as shoot the different ranges. I wanted to do it anyway to confirm drop data as well figure out the BDC reticle of the scope for quick reference if time to dial is not optional. This scope has the Boone and Crocket reticle and now M1 turrets. Hope it is reliable.

Thanks.

 

[Kennibear]

Do you realize how fine the adjustments have to be made accurately to move POI @ 100yds exactly 1"? Check the scope for movement per click. Use ten clicks round robin - ten up, ten right, ten down, ten left. Measure and divide.

I don't expect scopes to be dead nuts on POI changes. Consistent yes, but exact? No Way! Too hard to machine. Unrealistic expectation.

KB