Temperature, Humidity, and Accuracy

[TexasMLer]

I'm sort of baffled by the sudden lack of accuracy that I have experienced during my last two range sessions. The only thing that has changed is the weather. The temperature has dropped and the relative humidity has risen, but only slightly. Does the temperature and/or humidity play a role in accuracy? If so, how much and why would it be happening now? It has never effected my shooting before.

My patch/ball/powder combination has not changed at all. I can usually get a good clover-leaf group at 50yds from a steady bench unless I'm having one of those days that we all have. I'm pretty sure that the last two range sessions were not "one of those days". Not only have the groups opened up quite a bit, but they seem to have migrated from the center of the paper to just above and to the right of center. The strange thing is that the groups are consistently "off". Any ideas? Thanks!

 

[BrownBear]

I have never seen it done with BP, but years ago someone experimented with the effects of temp on smokeless powder loads in cartridge rifles. Don't remember all the details, but the velocity and pressure did go up and down with the temperature- enough that it was recommended that you not develop max loads in cold weather, then take a gun to Africa. Pressures could go over the top.

I was able to verify this a little with a couple of my rifles and a chronograph, looking at the same loads at 90 degrees and 25 degrees. Pressure signs can be subjective, but the chronograph showed a definite drop in velocity in colder weather.

I would expect BP to show a similar rise and drop in velocity and pressure with temp changes. I guess the big question would be whether a velocity change would cause the symptoms you are seeing. I know that slight changes in powder charge, along with changes in velocity, sure seem to change accuracy and point of impact, so there's something to think about.

In your shoes if I wasn't loading near max, I would be tempted to increase poweder charges in small increments to try to get the velocity back up to where it was in warm weather, in order to see if that affects your results.

 

[jtmattison]

I have seen changes in my groups with changes in the weather.
On warm, dry days my GPR shoots dead in the bull at 50 yards.
On warm wet days it shoots to the left.
On cold days it shoots high.
You have to shoot your rifle a lot and see how it performs in different conditions.
Since my shots range from ten to 50 yards when hunting the weather's effect on my roundballs flight are not important to me.
On the range however I do see a difference.

Huntin

 

[Keith]

Humidity can effect the wood in your stock. The wood will swell or shrink according to the humidity. The stock then pressures the barrel differently, thus changing the point of impact. This is possibly what is happening. The amount of sun on the sights and heat of the barrel can also effect your POI.

 

[TexasMLer]

Thanks very much for the very good information!

Outahuntin, I also shoot a GPR and my situation matches what you said exactly - Warm dry day=dead on...cold temp=group higher.

The group isn't so far off that it would really matter at the distances that I would make a shot on a deer which is 50yds maximum where I hunt. But at the range, while shooting paper, the difference is very obvious.

Thanks again!

 

[Mark Walker]

Are you shooting bp or pyrodex? I've had trouble with pyrodex in my rifles. The impact change could be wind, or bedding.

I'm running some humidity test at home.....probably wont come to a conclusion, but have a gauge above my bench for now. Just some thoughts...

Flincher

 

[TexasMLer]

Flincher,

I have used only Goex FFG. I've never tried Pyrodex, but if Goex becomes scarce, I might have to give it a try.

I thought bedding could be the reason for the impact change too, but I'm not sure if bedding matters much with such a heavy barrel and such thin wood. I'm no expert so I really have no idea. The gun did sit for almost 2 months without being fired, so maybe the bedding "changed", or something, while it sat?

Please post any results you do get from your humidity test. Sounds interesting. Thanks!