Cold weather and pressures

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ny32182

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Can cold weather increase pressures?

I ask because I was recently shooting some loads that I developed early in the fall. I know they were on the hot side, but no definitive problems. Well, I was shooting them again; exact same load; exact same rifle, but this time I was puncturing about one out of eight or so of the primers. The only difference I can figure is that the weather was 30-40 degrees colder.

I think I am going to back off some on that load anyway, but still curious about what is going on.
 
Well, I wouldn't think a Robinson would have excess pin protrusion. Generally, colder weather affects ammo the opposite way. Temperature sensitivity of powder usually reduces the pressure and velocity in colder weather.
Is your chamber completely clean and is the rifle going to full battery in the colder weather?
Just searching for answers to the problem.

NCsmitty
 
Hmm... to the best of my knowledge the chamber is clean/free of debris, etc. I did not have any rounds fail to go into battery/fail to fire, etc.
 
I have seen data that indicated pressure increases with very low temperature when i worked for an ordnance company, but this showed up only at -50-60f and greater and was explained as propellant imbrittlement that provided a higher surface area to the flame front; unlikely in your described case.
Can you look at the damaged (and/or undamaged) primers and the firing pin tip to see if perhaps your firing pin is damaged and that is causing the pierced primers?
Also, a LIGHT primer strike from stiffened lube or weak spring can affect the way a primer looks after ignition. Once your firing pin tip is damaged from a burn through, your primers go all down hill from there.
HTH
Doug
 
Unfortunately I have since processed the brass. The firing pin/tip appears to be fine.

Relatively light primer strikes are a possibility I guess. The XCR is known for striking relatively lightly in some cases, and my trigger has been modified and may be striking lighter than factory. I have never had a failure to ignite though, even shooting Silver Bear Russian steel case crap. I have no particular evidence that this individual rifle is striking lightly.

I suspect this is probably not it, since my lighter loaded plinking handloads all exhibit an entirely normal firing pin strike with the same primers.

The load in question is LC brass, 24.9gr Varget, 69gr SMK. Judging from my results and since comparing to some of my friend's loads with the same bullet and powder, I believe that one is on the warm side anyway.
 
The action pistol shooters talk about this being a problem with some of the powders they use, and I remember some discussion about this being a problem with an obsolete rifle powder called Hi-vel#2. I've never seen this effect in rifle ammo.
 
The primers are all seated fully inside the primer pocket (below the top of the case head)... I check each one by hand. The powder was Varget, all purchased in the last few months.


I have loaded up a few rounds which are the same, but are 24.1gr charge; I am hoping to shoot them soon and see what happens.
 
Clean your barrel with Copper Solvent just for the heck of it.

You might be surprised what built up in there since you did the load testing.

That will increase pressure fUrsure!

rcmodel
 
rc, that is one thing I could try. I normally just soak the barrel in Hoppes for a while. Is there a recommended brand? Thanks,
 
I don't know.
Everyone has their favorite it seems.

I have used Shooters Choice and Hoppe's Benchrest Copper Solvent with good results.

Butch's Bore Shine and Sweets 7.62 also comes highly regarded. I think it is a little harsher though.

Some of the new foaming bore copper solvents also get good reviews.

BTW: You could, as a test, leave Hoppe's #9 wet in the bore overnight, then run a clean patch through it the next day.
If it comes out green or blue, you got copper fouling.
#9 won't take it all out, but it will tell you if it's in there!

rcmodel
 
Last edited:
NORMALLY...The colder it is the lower the pressure will be. Just my guess. But cleaning out the copper is a good start. Like rcmodel, I use Hoppe's Benchrest Copper Solvent with great results.
 
For late season hunting, I developed loads at 20 to 25 F. I have hunted many times at -20 to -45F with a 6mmRem for deer and 222 for coyotes. I noticed a great reduction in terminal effects of the bullets, but experienced no over pressure signs.

Since the weapons were in the cold for a long period before firing, I understand that metal physically shrinks at cold temperatures. However, I do not how cold it has to be before it is a factor. I often wondered if the great reduction in terminal effects of the bullets phenomenon was cause by:
1. the temperature effects on the powders and/or bullet, or
2. if the metal was shrinking in the barrel differently than the bullet to constrict the bullet, or
3. if the bullet was shrinking more than the barrel to give a free-bore effect (this would produce higher velocities and reduced pressures at normal temperatures as the gas blows by the bullet creating a slight vacuum in front of the bullet).

Any ideas?
 
It has always been my understanding that flame temperature is reduced causing less gas expansion.

It could also be the very cold barrel acts to cool or condense some of the hot gas, causing a lower peak pressure.

I doubt seriously metal shrinkage has any effect whatsoever.

Hodgdon addressed the problem successfully by changing it's powder formula or additives/coatings in it's "Extreme" line of powders.

So it's fursure the powder itself.

rcmodel
 
The only powder I've ever had to spike pressure in cold weather was Blue Dot in a maximum .357 magnum loading...and I mean really spiked. July workup and January range trip was the difference between the cases falling out of the chambers and having to drive them out with a hammer and dowel rod. No other problems noted with Blue Dot in less than maximum charges.
 
The only other thing I can think of that will result in higher pressure in an otherwise known load, would be bullets seated so they engage the rifling? If you are shooting 69 gr SMKs at magazine OAL that's not likely but if you've inadvertently loaded some bullets long and you single loaded them so you didn't notice, those will exhibit a higher peak pressure. You could check your chamber with a dummy round by smoking or using a Sharpie on the bullet where the ogive starts and progressively seating the bullet further out until it is marked by the lands when chambered.
Your load at magazine length sounds like a fairly common High Power recipe. When I shot 69gr SMKs for High Power, I used LC brass and 25 gr Varget at 2.31" OAL. My magazine is modified and my chamber accepts this OAL. It's a load that I don't casually recommend but I shot over 4000 of them in MY AR without incident before moving to 75gr Hornadys for short range.
HTH
Doug
 
Thanks for all the info guys. Interesting even if I might still have a bit of a mystery (I will definitely try the bore cleaning with copper solvent... does it matter that my bore is chrome lined for that?)

As far as the OAL, all have been seated to max standard magazine length with very little variation (in the range of a few thousandths).
 
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