Temperature affecting groups

[Scratchthejeepguy]

Love the new title of this sub section by the way... Maybe I never noticed it before, but I think it may have changed...


Anywho...

With the winter months fast approaching, I usually hibernate into my recliner and don't venture out into the cold Wisconsin temperatures, but I thought this winter I might want to do a little distance shooting. (I say that now... Who knows what will happen when the snow flies though)

My question is this:
Does the temperature affect the grouping or just the POI?

I've found a good recipe for my R700 .308, and I'm happily getting .3-.5 MOA groups at POA right now. Once the temps start going down, (and if I do my job...) will my groups stay about the same size, just impacting a different location on target, and forcing me to adjust my turrets, or should I expect my group size to change also?

Assuming everything else stays the same as far as position, equipment, ammo, etc...

 

[shooter.dm]

I thank it would have a lot to do with the powder you use

 

[Scratchthejeepguy]

44.0 gr of Varget with 168gr A-Max

 

[Graham]

Temperature affecting groups

Here's an idea: Zero in the fall. Then go out on a cold winter day and shoot two groups, one with ammo at the outside temperature and the other with ammo that you've had in your inside pocket for an hour. Record the difference.

Then shoot alternate rounds at outside temp plus ones that have been left in a hot chamber for a few minutes. Record the deviation.

Ammo temp affects muzzle velocity. Different temps for each round equals larger spread equals larger group.

 

[Strykervet]

Damn Graham. Always chimes in first with the proper answer! I'll try to elaborate a little.

Yeah, I handload almost everything and experiment lots with loads. In 10mm where I prefer IMR 800x, a round loaded in summer vs. a round loaded in winter exhibit very different velocity --as much as 100fps!!! That's a LOT for a handgun. 1300fps pushing a 180gr. slug is about max (maybe over) in a full size pistol. 1400+ is damn right slinging the shit out of it and certainly over. You can feel the recoil and pressure signs become apparent too and it's sort of unnerving . I reckon it's why IMR 800x has systematically had the max load downgraded over the years. A lot.

In addition, USMC marksmanship teams have handloaders that take EVERYTHING into account, very anal, and actually heating the components and finished loads, then delivering those loads heated to the determined temp. for that rifle and shooter's preference for the shoot. So yeah, temp. can be a big deal.

You'll also notice differences if you shoot, heat up the barrel, then leave a round in there for bit. Not enough to cook off, but it will heat the powder and cause it to increase velocity, thus affecting trajectory, etc.

Some powders, as mentioned, are worse than others. I mention IMR 800x because it's a notorious offender here. Some are reputed to be extremely temp. stable but I don't have enough data to say one way or another --I think most of my powders are fairly GTG regarding temp. with the loads I use. Most. Temp. doesn't vary as much in WA as it does elsewhere around the nation either (but still has to be considered, particularly in dead summer months (weeks, really).

I don't like to load ammo that'll be used in different seasons to max loads for this reason. Better to have a safety window in that case.

Heat and resulting changes in air pressure can also affect things, to a greater or lesser degree depending, and is more noticeable at much longer range, but the effect it has on ammo is much greater in general.

Good luck, shoot safe.

 

[TroyMN]

Here's an idea: Zero in the fall. Then go out on a cold winter day and shoot two groups, one with ammo at the outside temperature and the other with ammo that you've had in your inside pocket for an hour. Record the difference.

Then shoot alternate rounds at outside temp plus ones that have been left in a hot chamber for a few minutes. Record the deviation.

Ammo temp affects muzzle velocity. Different temps for each round equals larger spread equals larger group.

Thank you for the info, I live in MN so my temperature change is large over the season. I am going preform the test you described when we get a Zero degree day. Can't wait...not.
I already have a load .5 grains lighter for temps over 80 degrees.

 

[Pinecone]

Varget will show VERY small velocity changes. 8 fps over 0F to 125F Velocity variation hot 8 fps. So small change due to temp, and very consistant, regardless of temp.

Other powders much more. Reloader 19, 94 fps 0F to 125F. Velocity variation hot 82 fps.

See here - http://www.hodgdon.com/smokeless/extreme/page3.php#top

 

[TimK]

Varget will show VERY small velocity changes. 8 fps over 0F to 125F Velocity variation hot 8 fps. So small change due to temp, and very consistant, regardless of temp.

Other powders much more. Reloader 19, 94 fps 0F to 125F. Velocity variation hot 82 fps.

See here - http://www.hodgdon.com/smokeless/extreme/page3.php#top

I've heard that a lot, but it's not been my experience. I had a great load worked up using Varget. Very accurate with no signs of pressure. The load was developed in roughly 50* weather. I shot the same load in the summer in 100* temps and it was so hot the primers were falling out. No other changes, just the temp of the ammo.

 

[Graham]

Varget gives me a tested one fps per degree in my .308.


Sent from my iPhone using Tapatalk

 

[smokebomb]

Looking at my logs from .308, I concur with grahams findings. At 5* F, 45.2grs varget pushing 175 smk velocity avg 2660 fps with ammo kept at that current atmosphere temp. At 90*F with same specs, velocity avg 2740fps.

90-5=85 deg range
2740-2660=80 fps variance

This coincides with my avg shooting environment temp of 55*F w/ velocity avg of 2705 fps

I assume smaller cartridges show slightly lower variance and larger capacity cartridges show slightly larger variance, but 1fps/deg F is a pretty close estimate.

 

[Goatman007]

Temp affects POI only. Cold air is more dense hence more resistance. At 800 yards I have a 12" drop at 0 Degrees versus 60 degrees.