Using temp settings in iStrelok and effect on calculations?

[wahoowad]

I’m setting up iStrelok for my new .25 Uragan / Athlon Argos 6-24x50 combo. After calculating my drop I clicked the ‘Additional Information’ link at the bottom and noticed a significant adjustment it made to my velocity. It said my adjusted velocity was 930 since my data for JSB Kings was using 86 degrees and iStrelok said my local weather today is 50 degrees. I am surprised to see such an adjustment for that temperature difference and it equated to 0.75” at 100 yards difference. Not huge but not insignificant when pesting birds. 

What settings do you have for your pellet data temp? I assume it means the temp you had when you determined your velocity and BC. For me I go with a BC I look up in one of the standard references although I know it can vary a little between guns. There is also a Zeroing Settings area which I am not using right now and is turned off. I’m thinking I might need to better understand when and how to use these various temp settings for shooting over 75 yards.

 

[Redick]

Where did you get that BC for the kings? Looks high the 50g jsb are .042 and the 44g are .033

 

[Glem.Chally]

Where did you get that BC for the kings? Looks high the 50g jsb are .042 and the 44g are .033

Looks like .25s not .30s but yeah seems high for kings. Should be closer to .040

 

[wahoowad]

I can’t remember where I got the BC. Any suggestions for a good reference for the BC’s? ChairGun says 0.036. Not using iStrelok Pro yet, don’t know if it has a pellet database or not. 

 

[Glem.Chally]

I can’t remember where I got the BC. Any suggestions for a good reference for the BC’s? ChairGun says 0.036. Not using iStrelok Pro yet, don’t know if it has a pellet database or not. 

I use .036 i got it from the pro database. The pro version is well worth the price had a fantastic reticle database also, constantly updated.

 

[cmatera]

Yes .036. I agree pro version is money well spent.

 

[wahoowad]

I'll update the BC and look in to Pro, but still no not sure about the temperature settings. But feel I need to as it made a big change in the adjusted velocity.

 

[Corvid_hunter]

I think air may be more dense as the temp drops, but I'm not 100% sure on that. If so, it would effect velocity.

 

[Michigander]

On the Strelok website they describe how to predict muzzle velocity if temperature is different from that when muzzle velocity was originally measured. They are mostly concerned with the effect of temperature on powder burn rates and pressures. I don't think this would apply to airguns unless you have muzzle velocity data for two temperatures. They supply a caculator for determining velocity change for every 15 degree change in temperature. If you don't have data specific to your rifle, they may be using a default correction factor that applies to powders with typical temperature sensitivity. Here is a screenshot of the calculator.

 

[wahoowad]

I wonder if I should set that sensitivity factor to zero since it is an airgun? 

 

[Michigander]

I would set it to zero unless you know differently. There have been several threads inquiring about the effect of temperature on airgun muzzle velocity. I don't know if any consensus was ever reached. You might try asking again. It would be interesting to see if there is any reproducible data out there.

 

[AirgunBill]

Temperature affects the velocities of all my pcp air rifles. Just go out and chronograph your rifle on two different days with a temperature swing and see the results. Then enter the information for the first two entries. One thing that I do is check my rifles velocity in my basement which averages 60 degrees this time of year. Then take the rifle outside and let it acclimatize while you set-up targets to check zero. Now the the question is will the Strelok program give an accurate prediction as it relates to air rifles. One thing before going out on a hunt where the temperature has change from you zero day you should check your zero. Bill