.308 Velocity Increasing With Temp

[M36]

I shot the other day when the temps were around 55-60 degrees. The average velocity from my load was 2699. I got a pretty good vertical spread and was happy with it. While researching projected velocities due to temperature, I found that on a 100 degree day, that velocity could climb up to over 2800 FPS. That sounds like to much and could turn into a to hot of a load with temp increase. I brought this out in another thread and decided to ask in a new one.

Could this turn into an unsafe load at that temp? Should I back off the powder and start over?

175 SMK
42.5 Varget
LC LR brass
CCI large rifle
2.180 OAL

Rem 700 SPS Varmint 26" barrel

 

[ksthomas]

M36,
It could definitely turn into a hot load, and this is pretty much true of any combination where large temp swings are involved. The nominal figure given for IMR single based powders is around 1.7 fps for every degree +/- from the baseline temperature. Double based powders tend to be a bit worse in this regard. And yes, there's normally a corresponding increase/decrease in pressures. Many of the newer powders that are being marketed as being less temperature sensitive improve this mark by various degrees, with most of the Hodgdon extreme series falling into this category. Varget is normally one of the least temp sensitive powders available. Not to say they're immune or exempt from these changes, just that they aren't as bad as many of the earlier powders. Work slowly, and take a wide temperature range into account when you're doing your load work ups. You wouldn't be the first guy to develop a nice, comfortable load at 60 degrees, only to find that it gives sticky extraction or loosens primers once it starts getting really hot on the range.

 

[ChemE1975]

Kevin, you mention a rule of thumb (1.7 fps per degree) for the IMR single base. Is there an accepted guesstimate for Varget?

 

[17874]

You won't see that much of an increase with Varget.

 

[ksthomas]

ChemE1975,

Not that I'm familiar with, but I have seen Varget used in some fairly drastic temperature testing certifications, and it's shown up very well. Off the top of my head I can't say just what it'd be, but I'd wager a good bit that it'll be well under the IMR figure of 1.7 fps per degree. Hodgdon may have this info in the chemical properties sheet of Varget, or may be able to give a rough estimate, but that'd be a question for the guys in Shawnee Mission.

 

[hondo1312]

http://www.hodgdon.com/smokeless/extreme/page2.php#top

According to this, varget has a temp spread of 8 fps from 0-125 deg F. So 0.064 fps/deg F.

Hope that helps answer your question.

 

[Donr13]

Develop the load in the hot months and you won't have to worry about over pressure in the colder months.

 

[M36]

http://www.hodgdon.com/smokeless/extreme/page2.php#top

According to this, varget has a temp spread of 8 fps from 0-125 deg F. So 0.064 fps/deg F.

Hope that helps answer your question.

Thanks hondo!

 

[40xs]

Typically, during a daylong 1000yd match, and even with varget powder, I come down sometimes a whole MOA in the late afternoon verses my starting zero in the morning. This, of course, is mostly in the warmer months of the year.

 

[Graham]

.308 Velocity Increasing With Temp

Varget gives me a tested 1fps per degree.

 

[hondo1312]

Varget gives me a tested 1fps per degree.

Why is there such a huge difference in your real world test versus what Hodgdon publishes on their data sheet?

I'm genuinely curious to hear what you think.

 

[Graham]

I don't know. I've tested my .308 Varget loads, from -20 to 115, and I get 1fps per degree.

 

[hondo1312]

Good enough for me. Thanks for you factual data!

 

[M36]

Out of curiosity, I checked two different speeds at two different temps with my Varget load and they work out to 1 FPS difference exactly. This gives me something new to pay attention to over the temp spreads that I shoot at to see if I have the same consistent results.

Yep, thanks Graham!

 

[gstaylorg]

Why is there such a huge difference in your real world test versus what Hodgdon publishes on their data sheet?

I'm genuinely curious to hear what you think.

I'm guessing it's in their best interest to advertise the least temperature sensitivity possible. My experience with Varget has been similar to what Graham posted, somewhere between 0.5 and 1 fps per degree in the temperature range from ~50-90 degrees F.