REVERSE TEMP SENSITIVITY????

[akmtnhnt]

You are correct.

When we build and test guns on the ground at near room temp, imagine how nice a cold they get when they have been cold soaked at altitude!?!

Temps on the guns that fly on aircraft go through horrific temp swings. Yet, the effects on the powder are more of a concern than those of the barrels/chambers.

Eh, total air temp is a lot higher than most people think especially with speeds around 1.0M

 

[cohunt]

This has been brought up before and the "cold constricting bore" thing was kinda de-bunked by a guy that actually knew the amount of "shrinkage" a steel bore encountered.

He said that copper and lead shrink at a higher rate than steel does so yes the bore does "shrink" but the bullet shrinks more than the bore does.....he actually had numbers to prove his theory.

That makes me think its gotta be a powder thing

 

[tonyf3]

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.

Am I reading this correctly.....
A temp stable powder will perform as expected and will be temperature stable, the increase in pressure and velocity is from the barrel constriction?

 

[ShtrRdy]

What kind of chronograph are you guys using at the cold temps? My Magnetospeed seems to act up when temps get below 32 F. Sometimes with higher velocity.

 

[akmtnhnt]

Am I reading this correctly.....
A temp stable powder will perform as expected and will be temperature stable, the increase in pressure and velocity is from the barrel constriction?

That's a hypothesis being suggested. Like one post above, I think you could figure that out with math and physics rather easily.

Just looking at the thermal coefficient of expansion of lead, copper and stainless steel, both the lead and copper is going to contract more than the steel - stainless or cromoly.

Given that, if you conducted an experiment to test this, I'd expect the opposite. That there is less pressure from copper/lead on the steel bore at colder temperatures than hot.

 

[Mikecr]

I believe there may be merit to temperature related dimension & elasticiy changes in barrels.
That stable powder is good but not enough.

 

[ButterBean]

RL19 and 22 both have this fault as well as many others

 

[RogerPA]

There was a thread on this forum entitled "Caution in the Cold", from February 2019, which covered this phenomenon. So much so that I've never forgotten it. An interesting read, especially if you're having trouble falling asleep.

 

[memtb]

In my case, everything used was at ambient for a few hours. After each shot, several minutes passed between shots…..so hopefully returned to ambient or nearly so! memtb

 

[Dean2]

Without getting into a long and complicated explanation of thermodynamics and the coefficient of thermal expansion for various materials let me provide the simple version. The coefficient is change per degree of temperature, per inch of length or area. While lead and copper have a higher coefficient than Stainless or Chromemoly, in the case we are talking about you have 1 inch of lead versus 24 inches of barrel. This length relationship also applies to the area of both. So simple example, a .308 bullet, sits inside a chamber that is ball park 1.25-1.5 inches in diameter and tapers to about .7 for the muzzle. Still nearly double the diameter of the bullet. Applying Pie x R squared you can easily see the large differences in area. The amount the barrel contracts or expands is exactly the same whether it is a solid piece or as in the case of a rifle hollow. In other words the hole constricts as much as the material.

The conclusion that the bullets area will shrink more than the barrels is incorrect due to the very large difference in mass, length and area.

 

[NW NM hunter]

Without getting into a long and complicated explanation of thermodynamics and the coefficient of thermal expansion for various materials let me provide the simple version. The coefficient is change per degree of temperature, per inch of length or area. While lead and copper have a higher coefficient than Stainless or Chromemoly, in the case we are talking about you have 1 inch of lead versus 24 inches of barrel. This length relationship also applies to the area of both. So simple example, a .308 bullet, sits inside a chamber that is ball park 1.25-1.5 inches in diameter and tapers to about .7 for the muzzle. Still nearly double the diameter of the bullet. Applying Pie x R squared you can easily see the large differences in area. The conclusion that the bullets area will shrink more than the barrels is incorrect due to the very large difference in mass, length and area.

Also to add more thought to this. I would think even if the bullet constricted more due to temp, it would be heated first from the propellant and friction in the start of the barrel and re expand prior to exiting the end of the cold barrel which is still constricted.

 

[Mikecr]

Bores expand with bullet travel as well. And cold steel is denser/stiffer, sometimes to brittle with a wrong alloy for our use (rare).
That's why I think it could matter.

 

[Muddyboots]

You are farther north so air is thinner.

 

[Fiftydriver]

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.

May be many things but likely its the cold barrel steel. When steel is warm, it has more elastic properties which means when a bullet is driven down the bore, the barrel steel will actually move around the bullet as the bullet travels down the bore and then return back to its normal dimensions after the bullet passes. When temps are very cold, steel will still do the same thing but it can not do it as quickly as warm steel. As such, it cant GET OUT OF THE WAY, quickly enough and as such, it takes more pressure to push the bullet down the bore. This causes pressure spikes. A solid bullet will exaggerate this issue. As will a heavier barrel. that said some barrel makers actually warn not to shoot in extreme cold temps because of the risk of barrel rupture….