Range Report Question about temperature affecting ballistics ....

[downrange914]

Hi - I'm just starting out in shooting, especially compared to the font of knowledge that is available here on this forum, but experienced something strange on the range last weekend that I was hoping I could get some feedback on.

Just to give some background - I'm shooting an AI AX 308 with a 28 inch barrel with a RWS 155g round - unfortunately I don't have muzzle velocity exact temperatures available or altitude - but if it helps I wasn't on top of a mountain or anything (!) - doing a very quick internet search the altitude where I was is listed at 60 but no unit measurement has been given (apologies)

I was shooting at 600 yards, and the MOA offset that I had previously got for this was 25MOA (100 yard is my zero and that is 10 MOA). This information had been previously collated through the use of something called a "zero range" which is approx 71 yards long and has a graduated target which allows offsets for various distances to be quickly calculated so you can at least "get on paper" for the distance you want to shoot at (but without having to go to that distance).

I was shooting early in the morning (around 8.30am) and the temperature was probably around 16 to 18 centigrade. The actual MOA I needed to get on target at 600 was around 22.25 MOA - no heat mirage was visible.

The ammo was left in a box next to me and consequently warmed up as the day itself did - by the time the range time ended (12.30) I would estimate the temperature to be around 22-25 centigrade (substantial heat mirage was visble) and I needed to adjust to 25MOA in order to hit the target.

Given this change is (very approximately) 3MOA which is equal to 18 inches at that distance - it seems like a huge difference to make just on the basis of a temperature change (although I am aware that the temperature will affect ballistics - and at least this change is going in the right direction - i.e. as temperature rises air gets thinner so bullet goes further - or so my logic says !) - but can anybody with the experience (or a ballistic calculator) please confirm if this sounds about right or whether I have something loose on my rifle please ?

Many thanks in advance.

 

[Lukas Koukal]

Ambient temperature influences both the external ballistics and the muzzle velocity of your round. I´d first get the input data straight and together - guesswork won´t let you see where it´s the equipment and where it´s just plain old you.

 

[Aimsmall55]

Could be several things. If its a new rifle the barrel may be getting "broken in". You will lose speed. Also brass retains cold as well as it does heat.... That being said what powder was it by chance??

 

[jbailey]

Something is wrong with your rifle or scope or something else.

Increase in temp, impacting both your ammunition and the air that the bullet flies thru, will make the bullet have faster velocity down range, and therefore require you to come-up LESS, not MORE like you observed.

And just looking at the range you were shooting at - 600yds - the impact of temparture is small (e.g. +/- 0.1-0.2mils), unless the temp change is extreme (like 0*F to 100*F). If everything was working right on the scenario you described, I would estimate you would have to move your come-up DOWN by 0.1mils to maintain center-hits, nothing like the UP 3moa change you observed.

Best of luck getting the rifle sorted out. I would start by making sure everything is still tight. If it is, re-do the shooting just to make sure the problem is still there.

 

[40xs]

I'm with jbailey on this one! all my experience in changes in temps. states that higher temps. calls for lowering moa. In the run of a day at matches i have to "crank" down at least an moa later on in the afternoon to stay centered on target, depending on how high the temps go. Your setting of 15 min. sounds spot on for hitting in the black at 600 however. I say revisit everything possible with your rig.

 

[Bob 964]

Things I have learned about temperature from members of this Forum:

As temperature increases, the air becomes less dense and your bullet will fly more efficiently and with a higher velocity. If you zero your weapon at 60 degrees F and fire it at 80 degrees F, the reduced air density will cause the bullet to fly faster and have a higher POI on your target. Remember, if temperature goes up, POI goes up; if temperature goes down, POI goes down.

Generally speaking, cold ammo doesn't perform as well as ammo at higher temperature. This is because powder burns at a faster rate when it is hotter and at a slower rate when it is colder. Higher temperature generates higher pressure, which generates higher velocity. Cartridges stored in direct sunlight will generally show higher velocities than those stored in the shade.

The accuracy of your barrel also varies according to its temperature. Any given barrel will start out at an ambient temperature, and heat up as it is fired, then reduce temperature as it cools. As with powder, higher barrel temperature generates higher pressure, which generates higher velocity.

Bad things happen when your ammo and barrel get too hot. First, the POI will shift as the velocities increase and your groups will open up dramatically. Second, your Extreme Spread (ES) and Standard Deviation (SD) values will increase dramatically. Third, the chamber will transfer heat to the loaded cartridge, which can cause pressure issues.

Keeping loaded ammo and your barrel at controlled temperatures is critical if you are going to achieve and maintain single-digit ES and SD values. I keep my loaded ammo in an insulated cooler, and I use the "blue ice" packs to keep ammo cool when it gets real hot outside. I monitor the temperature of the barrel and try to keep barrel temperature as consistent as I can when developing and testing new loads.

Hope this helps.

 

[eagernoskill]

What about Density Altitude

Hi - I'm just starting out in shooting, especially compared to the font of knowledge that is available here on this forum, but experienced something strange on the range last weekend that I was hoping I could get some feedback on.

I was shooting at 600 yards, and the MOA offset that I had previously got for this was 25MOA (100 yard is my zero and that is 10 MOA). This information had been previously collated through the use of something called a "zero range" which is approx 71 yards long and has a graduated target which allows offsets for various distances to be quickly calculated so you can at least "get on paper" for the distance you want to shoot at (but without having to go to that distance).
I was shooting early in the morning (around 8.30am) and the temperature was probably around 16 to 18 centigrade. The actual MOA I needed to get on target at 600 was around 22.25 MOA - no heat mirage was visible.
The ammo was left in a box next to me and consequently warmed up as the day itself did - by the time the range time ended (12.30) I would estimate the temperature to be around 22-25 centigrade (substantial heat mirage was visble) and I needed to adjust to 25MOA in order to hit the target.
Given this change is (very approximately) 3MOA which is equal to 18 inches at that distance - it seems like a huge difference to make just on the basis of a temperature change (although I am aware that the temperature will affect ballistics - and at least this change is going in the right direction - i.e. as temperature rises air gets thinner so bullet goes further - or so my logic says !) - but can anybody with the experience (or a ballistic calculator) please confirm if this sounds about right or whether I have something .....

If you were shooting at Bisley on Saturday or Sunday you had really funny weather - even thought it was warmer I also had to dial UP my elevation - though not as much.
There was heavy thermal activity towards Woking and we had really dense air on Bisley ranges that was moving from 10 to 1. We had some shots with 5 moa swings
So the DENSITY Altitude was very low - much lower than normal - so in affect you were shooting at a much lower Altitude Density.

Some good pointers posted above.

1. Also check your parallax on the scope and your cheek eye position. The combined effect may also create a different hit point - appearing like elevation changes
2. At 600 you also tend to shoot more rounds so be aware of how clean / dirty your barrels was - though you only note elevation not accuracy changes

 

[downrange914]

Guys - apologies for the delay in replying - had some other non shooting issues to resolve but back now !

Many thanks for all the knowledge - I will be investing in a torque wrench

 

[downrange914]

Ah - good point - yes I was shooting around 60/hour ...

I've decided not to eat for a month and bought a Stahlwille torque wrench so will find out shortly

Thanks again !