Range Report Temp affect on velocity

[dieselten]

I have a tikka t3 sporter in 6.5x55. I load a 140 amax over 48.5 grains of h4831sc in lapua brass. It shoots great, 24" barrel. I dont have a chronograph but have done a lot of research and was guessing my velocity at around 2800

I zeroed the other day at exactly 100 yards, dead nuts on, and it was 60 degrees. I shot it today at 460 yards and required 8.75 moa to come up, exactly. I measured the distance with 2 different laser range finders to confirm. It was 30 degrees today when I shot at 460. I did not confirm 100 yard zero on paper. The ballistic calculator says 8.75 drop equals 2690 fps.

My question is how much will a 30 degree temperature change affect velocity?

 

[TOP PREDATOR]

Re: Temp affect on velocity

if you confirmed your zero, you'ld see it would be off too, so the corresponding drop would be relative to that new "mystery zero".

what your experiencing is the excterior ballistics (weather / atmoshere) changes. maybe there's a little difference in the true velocity at the muzzle, but not enough to to really be concerned. it's what happens on the way from that muzzle to the target that is messing with your figures.

most of your emphasis should be on air temp, humidity, and altitude. depending on how much change is in everyone of those factors, it can cause a huge difference in your POI. 30 degress diff @ 460 yards can very well modify bullet flight / trajectory and where you print on paper.

i think we are missing something in your question: we don't know what your velocity was on the 60 degree day (as you were guessing on the FPS), or the amount of drop you were experiencing as compared to the 30 degree day.

 

[Lindy]

Re: Temp affect on velocity

The only way to really know is to how much temperature affects muzzle velocity is to chronograph the load in your rifle at different temperatures.

However, when I have no data on a load, the figure I assume is 1 foot per second per degree F. That's about what a good powder does. Some do a little better, and some do worse, but it's not a bad place to start.

Using that figure, with a 30 degree F. temperature drop, your muzzle velocity might be 30 fps lower.

 

[dieselten]

Re: Temp affect on velocity

Thanks guys and Merry Christmas!

I am just trying to get an approximation on velocity drop due to cold weather. The 1 fps per degree rule of thumb is very helpful.

Thanks!

 

[sticshooter]

Re: Temp affect on velocity

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">The only way to really know is to how much temperature affects muzzle velocity is to chronograph the load in your rifle at different temperatures.

However, when I have no data on a load, the figure I assume is 1 foot per second per degree F. That's about what a good powder does. Some do a little better, and some do worse, but it's not a bad place to start.

Using that figure, with a 30 degree F. temperature drop, your muzzle velocity might be 30 fps lower.
</div></div>that's about a dead on response.my load in .308 with pp 2000 mr averages about .60 fps difference per degree according to my chrono,so on average i would agree with the 1fps per degree theory for sure,plus he knows his stuff,Merry Christmas and good shooting.

 

[RemingtonSPS]

Re: Temp affect on velocity

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">The only way to really know is to how much temperature affects muzzle velocity is to chronograph the load in your rifle at different temperatures.</div></div>

Agreed. One thing i will add is that if you are using a ballistics program to determine velocity there is one other factor to consider. My brother once thought he had a load that was shooting 3,000fps and he was really excited until he found out his .25moa click scope was off. It was really a .275moa per click. So his trajectory seemed flatter than it really was. If you measure out an exact 100yds and test the tracking and click value of your scope it will help you when trying to figure out velocity without the cronograph.

 

[RemingtonSPS]

Re: Temp affect on velocity

I'll bet if your click values are say .235moa per click it will read that your velocity is low because it takes more moa to come up to zero.

If you haven't yet, check your scope clicks. If you enter in corrected values it will give you the right velocity assuming you have a correct BC

 

[Lindy]

Re: Temp affect on velocity

If you haven't read it, this may be a useful link:

Sources of Ballistic Program Inaccuracies