I know air temperature affects bullet velocity, but how much?

Yesterday I was curious about some loads that I had worked up previously, but had not chronographed. Both loads had shot less than MOA for 3 shots with no signs of pressure when I previously worked them up.

My chronograph is a F-1 Chrony set 10' from the muzzle.

Whenever I first set up my chronograph, I first shoot a few .22 LR bullets across it to see that everything is working.

When I got to the range yesterdsy, the sun was setting, and my thermometer read 4 degrees F, and falling.

I got set up and proceeded to shoot two Remington .22 LR's through my Rem 541 across the screens. This is the same rifle and bullets from the same box that I had previously checked my Chrony with. Last summer, these .22 LR bullets chronographed 1274 & 1256 fps. Last night they chronographed 772.5 & 772.4 fps. An average of 493 fps slower at 4 deg F than they were at 70 deg.

I then shot my 7mm Rem mag with 160 gr Accubonds. My powder charge is 1.3 gr more of H4831 sc than Nosler lists for IMR4831 at a book velocity of 3008 fps. I shot 2 bullets that chronographed 2885 & 2915 fps.

I then shot two 300 gr TSX bullets through my .375 RUM that chronographed 2789 & 2802 fps.

By this time, my fingers were getting numb from the cold so I packed up everything.

All three rifles shot slower than I expected, with the .22 LR's shooting almost 40% slower than previously measured. I'm sure both the 7mm RM and the .375 RUM shot slower than they should, but definately not 40% slower. I guess I'll have to put off any further testing until spring.

Cold does affect the pressure and velocity of your ammo but it shouldn`t be as much as you`ve seen in my limited experiance. I`d look at a cold battery to start with for the problem.

I have a CED chronograph. I checked the manual and it shows a operating range of 32-122 degrees (F) but also says if should operate down to 0 degrees.

Interesting observation. I do not have the answer but, four factors come to mind - considering it was a 22lr with a long barrel and very cold;

1/ barrel contraction due to the cold, tightening the bore, increasing bullet drag,

2/ lower chamber pressures - also due to the cold,

3/ bullet lube on a 22lr bullet that would probably get quite hard at those low temperatures,

4/ hardening of the lead bullet at low tempreatures, increasing bullet drag (in a tighter bore).

Might these combined factors cause the 40% drop in velocity?