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Velocity vs. Temperature

Started load development on a 6.5 WSM recently. Yes, in the cold! The velocities I'm getting are at least 200 FPS slower than identicle loads I found on Reloaders Nest.com. It was 28*F
when I shot ten loads from 62 grs. Retumbo to 67 grs. in half grain increments. Barrel length is 27". Velocity at 67 grs. was 3058. All were 140 gr. Berger VLDs. How much velocity drop is "normal" from say 70* down to 28*? Am I wasting my time developing a long range varmit load in the winter? I have the next ten loaded from 67.5 grs. to 70.8 grs.,pressure test)but wondering if I should wait for warm weather.
What say ye?
 
Hi Jerry,
For saftie's sake I would suggest waiting to develop a varmint load until the temp is at least 60 degrees. You can get a huge pressure difference if you have a max load at 30 degrees and shoot that load at 70 degrees.
Good Luck,
Jack
 
I would wait for warmer weather. Temp's do make a big difference in how a load reacts to pressure. When I was a young fellow I once worked up a hot varmint load for a 243 Win when it was in the 40* to 50* temp in the fall that was a real tack driver and in the summer when it was 80* to 90* it blew the primer. I had to back waaaay off the powder charge.
 
It's not the air temperature that is important, but the temperature of your ammo when you shoot it. Keep your ammo at a "summertime" temp and the velocities will reflect it. Use a cooler or keep them in your pocket.

Ray
 
Ambient air temperature will affect your point-of-aim as air is MUCH denser at cold temperatures & harder to push around, particularly the farther out your targets are. What Cheechako said is true that ammo temp is most critical, but it all adds up.

I try - very hard - not to test LR loads at any temp lower than about 50¡F.

When I was younger & more wrapped up in shooting I'd test OTC loads with a rig that kept my ammo at about 80¡F: metal garbage can with a hair dryer fitted into the side, just above the bottom. PITA to drag around, but it worked OK....
 
according to quick load you should really only lose 50fps or so going from 70 to 28. your about 75fps or so slow of what the estimates say for the temp
 
spclark is correct about air density but I assumed you were only concerned with MV and pressures, and they are not affected by air temp.

Ray
 
1. don't go by someone Else's posted velocity. 2) If you plan on shooting in sub freezing temps work a load only for those temps,separate from your summer loads, -40 to +30 degs. is a whole different animal than summer temps). 3. Every gun is different velocity wise.
 
CJ6 has good advice.

I read back a year or so about how certain popular rifle propellants behave when ambient temps are below 32¡F. With some, a -30¡ difference is enough to cause dramatic pressure spikes!

If you plan on shooting in cold conditions, it's sensible to prepare for those conditions differently than you would for something more comfortable... for a variety of reasons.
 

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