Cold temp caused faster speeds

[memtb]

I too have seen this "anomaly"(?), and described it a year or so back, in a previous post. Mine occurred with RL17. I always develop my loads on hot (90+) as to be certain that on a hot day, to make sure that my loads are safe in a high ambient temperature situation. We've always thought that pressures would increase when the rifle/cartridge were subjected to high ambient temperatures. My test was performed with ambient temperatures running between -3 to -4 F. The rifle and ammo were exposed to these temperatures for approximately 2 hours prior to shooting across the chronograph. While my increased velocity was minimal, it was completely opposite of what was expected! memtb

 

[BUSTINDOGS]

I work as a machinist and do a lot of aero space work were we need to hold very tight tolerances. I have literally made parts that were within tolerance at 78-82°F that were no longer within tolerance at 68.5°F. based on this i would think the increase in velocity is caused by the bore size becoming smaller and increasing pressures when cold. Based on my research the linear thermal expansion coefficient of 416ss is 6.4 and going from 68°F to 0°F would cause the bore diameter to shrink by .0002 I have worked a lot with 15-5, 13-8 and 17-4 PH stainless and based on my experience with these materials I would think the bore size would change by more than the .0002

 

[memtb]

I work as a machinist and do a lot of aero space work were we need to hold very tight tolerances. I have literally made parts that were within tolerance at 78-82°F that were no longer within tolerance at 68.5°F. based on this i would think the increase in velocity is caused by the bore size becoming smaller and increasing pressures when cold. Based on my research the linear thermal expansion coefficient of 416ss is 6.4 and going from 68°F to 0°F would cause the bore diameter to shrink by .0002 I have worked a lot with 15-5, 13-8 and 17-4 PH stainless and based on my experience with these materials I would think the bore size would change by more than the .0002

I think that I mentioned something similar to what you stated....but, it didn't get much traction! I'm onboard with it! memtb

 

[cohunt]

I work as a machinist and do a lot of aero space work were we need to hold very tight tolerances. I have literally made parts that were within tolerance at 78-82°F that were no longer within tolerance at 68.5°F. based on this i would think the increase in velocity is caused by the bore size becoming smaller and increasing pressures when cold. Based on my research the linear thermal expansion coefficient of 416ss is 6.4 and going from 68°F to 0°F would cause the bore diameter to shrink by .0002 I have worked a lot with 15-5, 13-8 and 17-4 PH stainless and based on my experience with these materials I would think the bore size would change by more than the .0002

I'm not a metallurgist, but this has been covered before by some "in the know".
Iirc they came to the conclusion that a cold bore to cold bullet will not create extra pressure as they did calculations that both lead and copper (bullet) will shrink more than the steel bore diameter will so theoretically you should have larger clearances/less pressure between bullet and bore when cold-- even so, iirc there are larger tolerances in the actual machining of bores and bullets than the cold Temps would change it anyway.
It comes down to other variables such as "bullet weld", powder temp instability, primer temp stability, powder lot variances, etc.
OP said he thinks he figured this out being a seating depth issue anyway.
Problem is there are so many variables that come into play its hard to prove which one is affecting the velocity, so testing and backing up with multiple testing data sessions is necessary to know what happens to your rifle/ammo combo in cold vs hot temps.
Some people have seen re26 spike above 85 degrees, others have not-- is that due to lot variances or other variables? Since there is no real practical way for us to chemically test powders we just jave to test and record data for our combo

 

[badthirtyone]

I Had a very similar experience two weeks ago with a varget load - which should be fairly temperature insensitive. Worked up a load 24 gr of varget under a 77 gr Sierra MK (super reliable/widely accepted match load). Did quite a bit of chrono'ing over the summer and fall (70 to 90 deg f), in a broken-in barrel. Shot it recently on a cold colorado morning and chrono'd considerably faster than expected. Good to see all of the discussion here about possible causes.

 

[Dean2]

Interesting read, lots of fancy and arcane reasons posited for this happening. I can tell you what we found for an easy cure, Try running magnum primers and if you are using magnum now step up to a hotter one. My guess is that the cold weather slows ignition, and it slows it a whole bunch at -40, and allows more build up of pressure, but that is just a guess. I do know that running hotter primers fixes it 90% of the time. I will add however that if 30 FPS increase in velocity is going from good to sticky bolt then you are already too close to max for a hunting load.

 

[DJ Fergus]

One possibility I've thought about, and I dont know if it's possible, is that the rifle barrel bore diameter is slightly smaller at colder temps. This would cause higher pressures to push the bullet out of the barrel and therefore higher velocity.

I've always thought about bore dimension tightening up in colder weather. I don't see how it couldn't tighten up. Everything else that was metal.... tightened up when it got colder. Steel shafts up to 1-1/2" diameter that bearings wouldn't go on until I froze the shaft is a good example.

 

[Dean2]

I've always thought about bore dimension tightening up in colder weather. I don't see how it couldn't tighten up. Everything else that was metal.... tightened up when it got colder. Steel shafts up to 1-1/2" diameter that bearings wouldn't go on until I froze the shaft is a good example.

Problem with that theory is that while the bore may shrink, so does the diameter of the bullet and cartridge case, and likely by more than the bore, unless you are keeping the ammo warmed somehow..

 

[DJ Fergus]

Problem with that theory is that while the bore may shrink, so does the diameter of the bullet and cartridge case, and likely by more than the bore, unless you are keeping the ammo warmed somehow..

That's a good point.

 

[Muddyboots]

Shortening the COAL will increase pressure on a round and probably increase velocity. The temperature of the steel barrel versus a copper jacketed bullet or even a mono is a wash since they will "shrink" at a great rate than the steel at colder temps. Over thinking this maybe when just normal reloading parameter of shorter COAL is likely root cause?

 

[ShtrRdy]

This is a great discussion. I experienced this sort of thing a number of years ago. One other possibility that came to mind is the chronograph causing an incorrect measurement. The electronics and/or mechanics could change to introduce an error to the measurement.

For example, in the case of an optical chrono or a Magnetospeed, under colder conditions, the sensors would be closer together due to the metal or plastic contracting. This would appear to the chrono that the bullet covered the distance between sensors quicker. Therefore the bullet must be going faster.

 

[BUSTINDOGS]

I did some digging and calculating and for a .224 caliber bullet in a bull barrel (1.2 straight no tapper) the barrel and lead bullet will contract the same amount. But what about bullet upset? the pressure behind the bullet should swag it to bore diameter. I'm not trying to prove that the bore being smaller is the end all be all, just trying to keep the thoughts rolling. Lots of things are changing, and lots of strange stuff could be happening...

 

[memtb]

Problem with that theory is that while the bore may shrink, so does the diameter of the bullet and cartridge case, and likely by more than the bore, unless you are keeping the ammo warmed somehow..

Dissimilar metals expand/contract at dissimilar rates! memtb

 

[BUSTINDOGS]

Dissimilar metals expand/contract at dissimilar rates! memtb

But the cross section of the materials are also different. Lead expands more than steel but the barrel wall thickness is greater than the diameter of the projectile.