How much does ambient temperature affect accuracy

[Metal god]

I'm not worried about POI shifts at different temps . What I'd like to know is if you have a sub MOA load at 70* is it likely the groups will open up at 100* ?

I'm shooting NM service rifle in 223 Wylde . 77gr smk , 75gr Hornady match , IMR 4895 . I've done initial load development and now I'm at the stage of confirmation . I've been planning on shooting on Sunday in order to confirm my initial loads were accurate with 10 and 20 round groups . Problem is we're going through a heat spell here and the temperature is supposed to be hitting 105° At the range Sunday . My issue is I don't want to waste the ammo if the results are not going to be what normally occurs in my area . It's not often it gets this hot here and even less likely it will ever be over 100* when I shoot competition .

I do like to shoot at different temps with loads I know work well in order to see what higher and lower temps do to the round . Problem is these loads are not confirmed yet and I'd hate to get faulse readings and dismiss a load that at the more normal temps would do well .

Balls in your court

[noylj]

Hotter air is less dense and less air drag to slow the bullet.
Hotter temperature and powder can produce more or less pressure.
Temperatures too low, and ignition is affected.
Accuracy, I would expect, is mostly affected by the shooter's suffering in high hear or freezing cold.

[g.willikers]

Imagine what it's like for folks living in places where it's 100 in the summer, below zero in the winter, and everything in between.
That's why charts are kept for all possible circumstances.
There's never a guarantee as to the weather conditions.
With the relatively small temperature variations where you are, it shouldn't take too many trips to the range to make useful charts.
That's the only way to know what your loads will be like.
Shoot them and see where they land, hot, cold and in between.
Don't forget when you go to a match, everyone else is in the same boat.
It all evens out.
It's not only the best shooter or reloader that wins, it's also who keeps the best records.

[waveslayer]

Just watch pressure issues that will effect the groupings and your safety. I use QL and place the temp at 90 rather then the default because out here I can shoot in the hot desert to freezing Temps when I hunt so I know my load was started at a safe level in the heat I never have to worry about pressure. I just have to zero my rifles for where I am hunting and the Dens altitude

[buck460XVR]

Originally posted by g.willikers:

Imagine what it's like for folks living in places where it's 100 in the summer, below zero in the winter, and everything in between

Kinda describes where I live. During the firearm deer seasons, it can range from 80 to 15 below. While I found that high powered rifles are affected ever so slightly as to not make much difference in POI, handgun calibers are a different boat. It may well have to do with the powders being faster for handguns calibers, or that the greater rifle velocities can vary more and still be relatively close to POA. Since I rarely shoot past 200 yards, it could also be a moot point. For my magnum hunting revolvers, I've tested a few powders and have settled on IMR4227 for .44 and .460 over H110/W296. It seems to be a tad less temperature sensitive and accuracy stays very similar in the extremes I tested it. It doesn't seem to make much difference in .357, but then that is a very short range handgun for me.

[Metal god]

Just watch pressure issues that will effect the groupings and your safety.

Good call , was not think of that . I have 2 AR loads and 1 308 bolt load I want to check . All of them are close to or at MAX loads . One of the AR loads had a couple pretty flat primers when shooting in the mid 80s . This is why I go back with 20+ rounds of my best 5 shot groups . If a 1/3 or more of these next loads show pressure signs I'd likely scrap them and drop to the next best group . In this case that group would not have the velocity I want . In that case I'd need to change powders . That just brings me back to my point . I'd hate to scrap a good accurate loads only because in the most extreme conditions it shows high pressure signs .

Remember I have only fired five shots of each of these loads . This next faze is to go out and shoot the loads that had the best 5 shot groups . This time shooting two 10 shot groups in each of those loadings . If for what ever reason they show pressure signs or group like crap I'd likely scrap them for ever or at least for quite a long time .

[Bart B.]

Having shot .308 Win's and 30 caliber maqnums in 20 to 110 degrees F without seeing any degradation in accuracy, it either doesn't effect it or I was too spaced out from cooking or freezing I didn't notice it shooting hand held rifles.

Benchresters often change their measures a click or two so dispensed powder charges change a tiny bit when temperature changes. Add a click's worth of powder if it's cooler, cut a click's worth when it's warmer. But their bullets are spun just fast enough for best stabilization at the slowest muzzle velocity their rifling twist will stabilize them. They are shooting groups 1/4th the size of mine in their competition compared to my discipline.

[Slamfire]

I think it is wise to test loads in hot weather. If the load blows primers, you know the charge is excessive. I have developed any number of loads, in nice cool 50 F, 60F, 70 F weather, only to have pressure signs in hot weather.

I am of the opinion that inherent accuracy is basically unaffected, unless you are blowing bullet cores out, but heat stress, that will ruin scores. I have not shot as well in hot and humid as I do in nice and cool. Can't get the heart rate down, one July, after lying out in 101 F weather at 600 yards, I darn near died from the heat. It was a killer. One other shooter claimed to be seeing double!

[Ifishsum]

I've never had it become a issue, but in hot weather I do try and keep my ammo box out of the direct sun as much as possible; I have seen guys even keep their ammo in a small cooler. I am sure that the bigger factor is my own personal discomfort, lying on a mat that feels like a frying pan, in a hot shooting coat with sweat dripping into the eyes on each shot.

[Bart B.]

Hot powder will increase pressure and velocity. The longer a round's in a hot chamber, the hotter the powder gets. Elevation zeros will change but accuracy won't except in extreme changes.

Shooting long range matches with .308 Win, for every 30 seconds the round stays in a hot chamber, I come down a quarter minute on the sight. With 30 caliber magnums, it's down a quarter every 20 seconds of chamber time.

If shots are fired at some regular interval but firing is delayed, the barrel will cool down enough that powder won't warm up to the same temperature it normally would; the bullet will go out slower. Which is why a sighting shot is allowed in NRA high power matches at ranges 500 yards or more if shooting is delayed more than 2 minutes with notno fault of the competitor. If delayed more than 5 minutes, two sighting shots may be taken.

There's been instances in long range matches where firing was delayed almost two minutes, then when resumed and no sighters were allowed, the next shot was a 9 out the bottom ruining a perfect score. The barrel had cooled down enough to not heat the powder up to the same level as the previous shots had when fired.

[F. Guffey]

The question comes up often, my advise? When developing loads for Colorado in Texas, keep your test loads in the freezer, take your test loads to the range in a cooler, then fire without leaving the cold ammo in the chamber long enough to warm up.

And we all should know when it comes to a successful trip to the lake for ice fishing: 'KEEP YOUR WORMS WARM!'.

F. Guffey

[Jim243]

^^^^^ I was going to tell you to use a cooler to keep your ammo in too.

I'd hate to scrap a good accurate loads only because in the most extreme conditions it shows high pressure signs .

Heat will affect your powder so keep them cool and out of the sun, I don't know about the freezer but definitely out of the sun. If you keep them in an inside pocket next to your body (out of the sun) all the time you will know that they are at 98.9 degrees F. each time you use one summer or winter.

Jim

[Bart B.]

Those cartridges will be lower than body temperature. I just measured the temperature of my T-shirt under my regular shirt in a room temperature of 72 degrees. T-shirt temperature is 85 degrees. Outer shirt temperature is 80 degrees. Outdoors in 32 degree weather, cartridge temperatures will be even lower.

[Metal god]

Just so you guys know , I did not go shooting on Sunday . I did not want to do load development in that kind of heat . Jokes on me though . Forecast was 103* and it never hit 90* oh well

[Slamfire]

Hot powder will increase pressure and velocity. The longer a round's in a hot chamber, the hotter the powder gets. Elevation zeros will change but accuracy won't except in extreme changes.

You are smarter than all of Army Ordnance. In 2010 the Army first article tested a SAWS. The test report required the SAWS and the ammunition to be conditioned for three days at 160 F. Apparently the technical core competency of the Army is so low, that no one understood that heating ammunition raises combustion pressures. Firing 160 F ammunition in a 160 F weapon caused malfunctions. Later analysis shows that they were firing 72,000 psia ammunition (predicted) in a weapon designed to support a 52,000 psia cartridge (most likely design load). So what did the Army conclude was the problem?: Oil!. Yes oil, not overpressure ammunition. Lubrication’s Contribution to Case Failure www.dtic.mil/ndia/2011ballistics/11826.pdf

They still don't understand why they had case head ruptures!

[Clark]

I remember when Bart first posted that info decades ago and before www gun forums.

But now there are temp stability coatings. I have read posts by guys who took ovens and coolers to the range and chronographed the new powders and the old temp sensitive powders. They have verified Hodgdon's and Alliant's claims for their new powders.

And that is why it took me two years to find this jug of H4350 on the internet. The word is out.