Effects of ammo temperature itself - not air temp

Discussion in 'Rimfire and Airguns' started by top predator, Mar 27, 2011.

  1. top predator

    top predator Well-Known Member

    Nov 17, 2008
    We all know how air temp and humidity effects the flight of the bullet, but what about the temperature of the ammo itself - regardless of air temp?
    While I still had some cold weather left, I thought I'd try to see what would happen if the ammo temp rather than air temp would have an effect on POI or other factors. The morning was cold, air temp of 16 deg F. I used match velocity and high velocity ammos, left them sit in the shade outside for 30 min. I then put the same in magazines and left them on the vehicle defroster (couldn't get a temp reading, but magazines and ammo were quite warm, almost hot to the touch).
    I shot 1 ten round group of "cold" ammo, then shot 1 ten round group of "hot" ammo, both through a sporter barreled and heavy barreled Savage MKII from the bench at 50 yds, rear bag and bipod. Wind was light, 3-5 MPH from 3:00, low humidity, sunny, snow covered ground. 1" circles were the target. When I switched from the match velocity ammo to the high velocity ammo, I dry swabbed the barrels, then shot 10 rounds off target before the test groupings to get as much of an equal fouling / seasoning as I could.
    Although in hind sight I should have shot them through a chronograph, I didn't (as i didn't anticipate a need for it at the time) but should have.
    Results of the High Velocity CCI Blazer ammo, MKII pencil barrel top, MKII heavy barrel below:
    Results of the Match velocity Wolf Match Target ammo, MKII pencil barrel top, MKII heavy barrel below:

    As a slight disclaimer, the CCI Blazer ammo was weight sorted, the Wolf Match Target was not. In general it seemed that both type of velocity ammos grouped better when the ammo temperature was higher, and also produced a higher POI, even though a slight one. I can only imagine that at 100 yards and further out the difference would even be more pronounced, and stayed with the 50 yard results.
    I hope to try the reverse this summer - high air temperature with the ammo sitting in the sun outside VS cooler ammo sitting on the defroster with the air conditioner on.