Velocity and temperature

[HolyMeekrob]

I've been working with 77gr SMKs in my AR lately and found a well-behaved load at 2485fps. This ammo was loaded and fired in approximately 30*F weather, but I have no experience in how loads change as the temperature begins to climb outside.
I expect the velocity and pressure will increase, but how much? And how about effects on ES, SD, and group size? I understand there's probably no predictable pattern, so I'm just looking for a rule of thumb.

 

[LCazador]

Re: Velocity and temperature on ballistics

I have had experience with this , and the results can be drastic if the temp change is above 50 degrees of change . Regardless even a change of 25+or- degrees is going to change everything . Ballistic coefficeint is affected by changes in altitude , temperature , barometric pressure , and humidity . Powder charges will also be affected by all the above . I have always tried to work my loads so I will not have more than a 20 degree difference . Sometime back I worked up a nice load using the 69 Sierra MK's , at a temp of 60 degrees at an altitude of 2000 feet . That load at 5500 feet and at 95 degrees blew primers all day and was unuseable for the rest of the trip .

 

[HolyMeekrob]

How far did you have to step back your load to bring down to workable pressures? Also, did you notice changes in grouping or velocity spreads?

 

[RonAKA]

It all depends on the powder you use. What is it?

 

[HolyMeekrob]

I'm running 23.6gr of Varget in this load

 

[RonAKA]

Varget is one of the Hodgdon Extreme powders, and very temperature insensitive. I would not worry too much about loads developed at low temps and shot at high temps.

 

[JLDavid]

I agree with RonAKA. I have tested loads at 20* then at 50* and at 77* with H4350, Varget, H4831 and H4198 and have not found an excessive increase in pressure or velocity. That being said, each gun is a different animal so I would do some testing as your temperature increases and develop your own rule so that you can keep your AR shooting in that node. You have already established the velocity to shoot for. Good luck and keep us posted.

 

[LCazador]

The load was stepped back about 10% and the entire load had to be reworked , as it affects group size , velocity , etc. . The powder I was using was Varget . Hodgdon only states that their " Extreme " powders are less sensitive to temp changes , FYI . I had the same experience with 4831sc in my 6.5/284 .

 

[RonAKA]

I was having some concerns about temperature with Varget and posted a question here about it. I received an excellent response from Kombayotch. See this link:

http://www.accurateshooter.com/forum/index.php/topic,3754350.msg35894541.html#msg35894541

I'm now convinced that the variation I was seeing was due to my chronograph inconsistency.

 

[LCazador]

There is no powder , and no round that will handle any and all temperature varients the same , ET Tu Brute !