REVERSE TEMP SENSITIVITY????

[Calvin45]

Hey all, I brought this up in my imr 4831 thread but it deserves its own thread.

Anyone here observe such a thing as reverse temp sensitivity? Cuz I have and it puzzles the heck out of me. Like the Grinch who stole Christmas I've puzzled and puzzled till my puzzler was sore!

Both loads in 300 win mag.

One is 8133 under a 225 eld m

The other is superformance under a 120 Barnes tac tx.

Bullets and bore are hbn treated

Both loads worked up in the summer months in Saskatchewan. Figured that's be a guarantee of good to go in winter, just might lose a little velocity. Nope. So weird.

Two winters now this has happened, and last summer the loads were absolutely fine again so it's not cold welding or any other nonsense about shelf life.

In the summer, like 27 celcius or 80 Fahrenheit I think, it was a 120 tac x at 4050 fps. In -17 or right around 0 Fahrenheit or so, it was 4160 fps and I had to tap the bolt open!!!!!

Same conditions with the eld m 8133 load and in the summer it's 2760. In the winter it's doing 2830 and the bolt is ever so slightly sticky.

And I've done this with other bullets treated with hbn and different powders, doesn't happen, got nothing to do with hbn.

Doesn't happen with any of
The other powders I've used.

Experiences and theories welcome, let's shoot the breeze and maybe we'll learn a thing or two along the way.

 

[memtb]

I had similar with RL 17 in my 375 AI. I only do load development in the summer…..preferably above 90 F. I did some shooting across the chrono with all components (rifle/ammo sat outside for a couple of hours to stabili at ambient) at a few degrees below 0F.

Similar to you, I gained velocity…..though, only about 10 fps! memtb

 

[Pdvdh]

Yes, I've seen some powder / bullet combinations increase slightly in MV as temperatures decline. Not drastically. But some increase in colder temps.

I came to the conclusion it's a case where the powder manufacturer has added too much pixie dust, during the powder manufacturing process.

 

[Calvin45]

I had wanted to sneak out during a truly cold snap here this winter to test loads just for research. It gets down to below -40 (and c and F meet around there) a few times every winter usually. Will admit I'm not sure i feel safe doing that anymore with the superformance load.

 

[Pdvdh]

I've tested down to -25F here in Alaska in the past. Just to collect truly cold temp versus MV data. Colder than I'd be hunting in.

 

[Dmagna]

It's probably not the powder. When steel gets cold especially really cold, it contracts. When this happens to a barrel it could cause pressure issues and increase fps do to the barrel. Could be a barrel issue

 

[Mark37082]

There are hearty people here. -40F or even -0F are a no go for this southern guy. At those temps I would agree with Dmagna. There are so many things that just contract at these extremes.

 

[Pdvdh]

Lots of folk have reported increased MV at colder temps. Some have only chilled their ammo down to cold temps. In fact most haven't chilled their barrels. Most seem to only chill their ammo.

My powder temperature sensitivity testing always involved both barrel and ammo cooled down to the same temperatures. When testing during the summer, I've placed both rifle and ammo in the deep chest freezer overnight to test @ -5F during outdoor temps of 50F.

But having reviewed many many threads over the years on this subject (going back to 2005), I conclude most guys only chill or heat up their ammo when testing powder temperature sensitivity. Too much time involved for most to wait to chill their rifles down. No patience...

 

[Dean2]

Have done a lot of shooting below -20F. What you have observed is actually quite common. I have never had a load give me high enough pressures to pierce a primer or stiffen the bolt but I have seen them pick up as much as 150 FPS. Stainless barrels show this more often than Chrome Moly because the rate of contraction on stainless is higher in the cold. For a few years there was a belief that you should not shoot stainless barrels and actions at temperature below -35 F but I have used them to -50 with no issues.

The powder slows down in the cold but the contraction in the barrel diameter is more than enough to offset that. What you will find is if you fire a string of shots fairly quickly, the velocity will drop back down as the barrel heats up.

 

[tonyf3]

I had a 7 RemMag shooting 180 grain hybrids under 72gr of Retumbo. This load was developed at 5000' at roughly 55-60 degrees F.
A few seasons later i was target shooting outside of Great Falls. 0 degrees, snowing, 4800'.
No chrono, but I was impacting about 3/4MOA high at 1,000 yards. On JBM and Applied this averages about 40fps faster in order dial 3/4moa lower. My notes from that day also point out that recoil had a different punch to it.

I tried an experiment this summer.
I had some load dev to do in the Sacramento and had lots of 90-100 degree days.
I put some dry ice and a block of ice in a small cooler and put the ammo the cooler. Ammo was 15-22 degrees
I didn't get a spike in velocity.
I got alof of funny looks and alot of questions so I don't think i'll do that again.

 

[Calvin45]

It's probably not the powder. When steel gets cold especially really cold, it contracts. When this happens to a barrel it could cause pressure issues and increase fps do to the barrel. Could be a barrel issue

Okay but why would it not do that with other powders, just these two?

I have tested a good many others.

Of course it could just be that these were closer to ragged edge than all others to
Begin with but that's very unlikely.

 

[Pdvdh]

Okay but why would it not do that with other powders, just these two?

I have tested a good many others.

Bingo!
Because it's the powder temperature sensitivity. Not primarily a barrel temperature caused phenomenon.

 

[Calvin45]

I will say, someone on the other thread mentioned "flattened ball powders" being less stable in extreme conditions, and I do know from working with ball powders (the old h870, win 760, leverevolution, can't remember what else) in the past that they do some weird and unpredictable things when it gets very cold.

 

[Dean2]

Okay but why would it not do that with other powders, just these two?

I have tested a good many others.

Of course it could just be that these were closer to ragged edge than all others to
Begin with but that's very unlikely.

The MORE temperature stable a powder is, and 8133 is an Enduron line powder as is Superformance, the more likely you are to see the speed up effect. Non-temp stable can lose enough speed to offset the barrel contraction, VERY stable powder doesn't so the barrel contraction shows up.