Temp sensitivity in 7mm mag

[ericbc7]

I bought a new "old stock" 7mm Tikka and set it up for deer season at the last minute.
I sighted it in at 100 yds with federal 150 gr ammo and tested it at 461 yds on steel, translating to 25 clicks on my Athelon mrad scope. Temp was about 50 degrees or more (ammo was warm from cabin). After 2 days of hunting with rifle and ammo in truck (so ambient temp) I decided to check my zero on the 461 yd target with temps at 7 deg f.
First shot missed steel entirely, I had to add 5 clicks to the elevation to get back on target. Using StrelokPro pro and by experimenting with inputting lower velocities I estimate I lost over 300 FPS due presumably to temp change. I assume this will vary depending on powder type or brand of ammo.

the takeaway is don't assume your drop charts apply when weather changes dramatically. Btw, it got down to -3f but I didn't check zero due to time constraints.

 

[Len Backus]

Moved to more appropriate sub-forum

 

[ericbc7]

Moved to more appropriate sub-forum

Which sub forum did this get moved to?

 

[fiftybmg]

Powder type does make a difference, which is why the manufacturers have come up with temperature stable powders like the IMR Enduron range.

To be sure, chrony the loads at the two temperature extremes, and if it's important enough, make a drop chart for each.

A drop chart is meant to be for a particular set of ambient conditions. When the conditions change, the drop chart changes.

 

[Len Backus]

Which sub forum did this get moved to?

Rifles, bullets, barrels and ballistics

 

[Canhunter35]

I would check your 100yd zero

 

[fiftybmg]

Another thing - that 300 fps calculated velocity difference doesn't sound right, hence the suggestion for chrony verification, and looking at possible scope malfuntion.

Your load should be around 3000 fps from the factory. +300 fps would stick the bolt and stretch the primer pockets so far that the primer would be blown out.

If you put some cartridges in a freezer, and some on your dash in the sun on a warm day, the difference in velocity is expected to be around 3 percent, provided the powder is temperature sensitive.

 

[lancetkenyon]

Are you sure the ammo is the same? From the same box?
The worst powders for temp sensitivity are only about 2fps per 1°F swing. No way you lost 300fps in 50° temp swing.
I would look at a scope/mount issue, parallax issue, shooting position/clothing issue, action torque/stock issue, etc.

 

[Canhunter35]

Are you sure the ammo is the same? From the same box?
The worst powders for temp sensitivity are only about 2fps per 1°F swing. No way you lost 300fps in 50° temp swing.
I would look at a scope/mount issue, parallax issue, shooting position/clothing issue, action torque/stock issue, etc.

Since the scope was just set up, my bet is it slipped into position from firing or bouncing around in the truck, mounts should retorqued and confirm 100 yd zero.
To see mv drops like that you need to be hunting in the -40f temperatures