Temperature Sensitivity

[Fox Tales]

Hi All,

 A couple of questions about powder Temperature Sensitivity .

  Is temperature sensitivity in powders proportionate ? For example if a drop from 15°C to 0°C results in a drop in velocity of say 150fps would each degree equate to 15 fps ?

 It's possible to enter powder temperature sensitivity into Strelok Pro to compensate but with a substantial loss in velocity may come a loss of accuracy . Does anybody load seperately for Summer and Winter ?

 Just mulling this over . Hoping to gain a little knowledge and understanding. 

Cheers All

[DaveT]

I use chrono data to check mv variances but treat it as linear / predictable for convenience with Applied Ballistics.

You could be chasing detail all day otherwise.

I am lucky enough to have a Labradar so its no hardship to set it up to verify mv on the day.

I don't often see a need to have Summer / Winter loads in the UK as our weather is reliably shite all year round but I have seen the odd pressure spike on really exceptional days if I let a round sit in a hot chamber so avoid that practice.

 

[gbal]

Temperature changes will affect velocity and Point of Impact (and pressres).

But not as much as the OP suggests...there is some quite deent research on this. Eg:

The NRa tested IMR powders and found an average figure of  1.7 fps mv change for every 1F degree change (increase/increase or decrease/decrease).

Remember US shooters may experience more extremes of temperature than UK shooters,both low and especially,high (non ignition is an issue for very low temps there).In summer heat,exposed cartridges might reach 130*F in direct sun,which is unlikely on Dave's weather summary here.  OK sample efffects:

A 243 sighted in at 40 *F for  deer season,used  for prarie dogs  up to 1308F-plus 136 fps MV

243 100g @ 2850fps might show an changed POI at 300y (100zero) from 14" lowcold to 11.5 low  when hot).

Accurate Shooter July 4 2017  reports testing by Sierras chief ballistician:

he loaded 30 cal 175 SMK with 'a suitable commercial powder',below max.

He then shot ten idetical loads at 200y ,in retrurn to battery rig,on a day whose range temperature was 70F,in each of three cartridge temperature conditions-

1)from a freezer at 25.5F which averaged 2451fps mv

2) heated   to  70F which averaged 2500 fps mv

3) heated up(electric lamp) to 130.5F  which averaged 2596 fps

Groups were around 1/2 moa  for 5 shots,with very small improvements with temperature. (.3 inch max)...which is good-precision was not significantly impaired here.

But the  F130.5 load changed point of impact relative to the other two (the same) by about  an inch and a half (and it was low-despite being 96fps faster mv than the 70F  POI)

There were no undue pressure signs-remember this was a sub max book load.

OK,allowing for the considerable differences in powders ( and some are by design less sensitive to temperature change) and the limited one load sample etc ,the take away from all this sems to be that:

1)Temperature changes in ammo in the chamber (atmospheric temp/conditions are separate  varables to ammo temperature) will result in MV changes,of the order  1-1.5 fps increase for each F degree increase (calibre  etc will vary a bit, with the higher value for higher temperatures, and probably near enough linearly in the mid range for UK (1 fps for 1 degree F won't be too far out,simples-but check your loads-data was for 30cal 175g.Less temperature sensitive powders will reduce this ball park figure considerably,as will other factors).

2) There will be likely POI changes,and probably not predictable,though unlikely to be in excess of 1/2 moa  at 200y-clearly, this should be checked for each load/rifle by actual shooting.

A simple insulated box/container would serve quite well in UK to keep temperature of ammo going into chamber a bit more mid range. Winter stalkers have pockets.

good shooting,weather warm or bracing.

gbal 

 

 

 

[TJC]

I record MV/Temp all the time and even took a batch of handloads to Florida recently to help gather more data. 

[VarmLR]

The only way to test each powder batch is to record your own MV data using the exact same loads in differing temperatures.  Strelock pro has a function which allows you to enter this data and it then works out the temperature sensitivity, automatically applying it to the output for future reference with that load.  I've found that due to ES, it's best to record this data at no less than 5 to 10 degree jumps usinf strings of no less than 10 shots to get the average.  Different powders react differently.  RS seems less sensitive than some others I've used.  Vhit N140 isn't meant to be too temperature sensitive but on a few really warm days, it has surprised me with just how much additional velocity( and pressure!) it develops, so it's important when load developing this time of year to take things like this into account to avoid over-pressure issues later in the year.

[Fox Tales]

 Hi Guys , 

  Thankyou for your replies .

 The figures  I used where quite clearly just made up for ease of explaining the question. 

 I'm on top with most parts of Strelok pro but haven't used the temperature sensitivity settings, quite clearly I need to , to get the best from the App. My original loads were chronographed on a mild 15°c . I shall get the chronograph out again on a cooler day .

  Thanks again and DaveT you're spot on with the weather, you must surely work for the MET office. 

 Cheers All 

 

[MJR]

TJC - great data!