Different altitude different groups?

[nmkid]

Recently moved from VA. to NV. I went from around 200' above sea level to almost 4700' above. I took my great shooting 25-06 out to the range and a rifle that had always shot .750 and less went to 1.250. Torque settings were all checked. I was using:
WW cases
CCI 200
IMR 4064 50.5 grs.
Hornady 75 gr Vmax.
Nothing changed except the altitude. Three of us guys shot it and neither of us could post under 1.00. This was at 200 yards. One thing that comes to mind, the temperature was at least 20* warmer than when the load was concocted. What say you guys?

 

[Fuj]

Get yourself a ballistic calculator. JBM, and Hornady, and others have
them online free to use. One of the boxes that always needs filled in
is the elevation.

 

[davidjoe]

I’m not aware of any characteristic of denser air at lower altitude that can make the gun shoot a tighter group relative to less dense air.

I think you’d have to rule that out as the cause for bigger groups. The absence of all air would permit a gun to reach its full accuracy potential, so the spectrum is no air at one end and say, water, at the other end and obviously we shoot in the middle zone.

Denser air has the following effects and they are all accuracy-negative: less bullet stability for a given twist rate, lower velocity, faster deceleration, greater effect of wind drift for a given wind velocity.

 

[doctorh]

I Also move from va. To nv. True the avg. Temperature is a lot higher,especially summer time. I found that my reloads that worked perfectly in va. Were way hot where I lived in nv. More so in the hot summer months. I had to back off 3 to 5% In powder charge to keep same velocity. Need a chronograph to check velocity. Back off appox. 2 grs. And see what happens. Doc.

 

[K22]

Since I shoot (at ranges) and hunt all year around, I have noticed some POI changes from summer to winter months with the same loads in the same rifles. Since I long ago moved from ball to extruded powders, I suspect this is due to changes in air density not temperature effects on powder stability. However, the groups are not significantly different, but the vertical does change.

I have noticed that at one of the clubs where I shoot, the groups are always better than the other club where I shoot. The one with the best groups is banked with heavy wooded tree lines which minimize wind effects and mirage whereas at the other club the range is open with no wind breaks and receives direct sunlight most of the time.

 

[Cecil S.]

Could bullet weight come into play in this condition? You may need to start all over as your normal atmosphere is way different. Just my opinion as a newbie. Following this for the information!

 

[nmkid]

Think I'll try some heavier bullets and maybe some 4350/4831. Probably go with HHT bullets at 95 grs. I gave the rifle to my nephew and he is in California so, copper bullets for hunting.

 

[Clancy]

There’s a reason guys show up days before a match to tune. Although, altitude has a lot less to do with it than the climate itself.
Plan on redoing most or all of your loads, no way around it. Welcome to the West.

 

[gunsandgunsmithing]

I’m not aware of any characteristic of denser air at lower altitude that can make the gun shoot a tighter group relative to less dense air.

I think you’d have to rule that out as the cause for bigger groups. The absence of all air would permit a gun to reach its full accuracy potential, so the spectrum is no air at one end and say, water, at the other end and obviously we shoot in the middle zone.

Denser air has the following effects and they are all accuracy-negative: less bullet stability for a given twist rate, lower velocity, faster deceleration, greater effect of wind drift for a given wind velocity.

In my experience and in my part of the country, temps are a lot bigger factor than air density to tune. But I don't doubt anyone that tunes by air pressure, especially in places that don't see wide temp swings throughout a day or even seasons. Bryan Z has done a good bit of testing mostly around barometric pressure and has proven a very statically significant relationship...in western Washington state. He says the temps generally stay very stable as compared to most places but he does see pressure differences, so it makes sense to use that data in his area, maybe more so than temp. Here, I find temp to be approximately 4x more value, so considerably more important but but factors matter. FWIW, when testing pressure changes, I much prefer station pressure as opposed to density altitude or barometer. I don't see any value to humidity unless you're loading ammo outside. If it matters in a "sealed" case, the difference is lost in the noise of bigger factors and frankly, it's difficult to quantify when other factors are changing at the same time, outdoors. Some have tested by using powders intentionally dried to specific moisture contents but I don't recall it being substantial on its own. I'm certainly open to discussion with anyone that has tested it by this or similar method more than I. Point being Barometer and DA are both adjusted numbers factoring humidity and altitude into the numbers. Station pressure is raw, unadjusted..the weight of a cubic ft of air, where you are standing. Some Kestrels measure station pressure but I find them to need one more decimal place of resolution for best data. There's usually a local weather nut just about everywhere ya can go than has a weather station in close proximity that broadcasts that data online if you google foo for it. They typically do show better resolution...fwiw.

But, as I've often said...I use group shapes to read tune and all of these other numbers are only good if you can chart them and find accurate correlation on the target. Problem is, IME, temp and station pressure get us ALMOST there but there must be other factors, as I've not been able to chart it accurately enough on target to rely on 100%. Still worthwhile but ya know, group shooters and anyone that tunes at a match for conditions have been looking for a 100% formula for decades across thousands of shooters. If it were that simple, I have to believe that someone would've figured it out a long time ago. But the target never lies and that's ultimately the only thing I've found to be 100%. Not saying it's impossible but there's potentially multiple missing pieces and it's tough when other factors change too, creating a moving target.

 

[JFrank]

Recently moved from VA. to NV. I went from around 200' above sea level to almost 4700' above. I took my great shooting 25-06 out to the range and a rifle that had always shot .750 and less went to 1.250. Torque settings were all checked. I was using:
WW cases
CCI 200
IMR 4064 50.5 grs.
Hornady 75 gr Vmax.
Nothing changed except the altitude. Three of us guys shot it and neither of us could post under 1.00. This was at 200 yards. One thing that comes to mind, the temperature was at least 20* warmer than when the load was concocted. What say you guys?

Sounds like your rounds are running a little faster in the thin air and starting to move out of the node. I would run an abbreviated charge ladder to get back into the node, might only be a tenth or so difference.
No big deal .

 

[Clancy]

@nmkid you may find your favorite powders change as well. Lots of what works in the east doesn’t work as well out west. Just takes some getting used to.

 

[6ShotsOr5?]

My first thought was that you’ve got more wind. Use a Chrony to rule out MV variability. Ambient temperature climbing or ammo in the sun. How long the round is in the chamber before firing.

 

[nmkid]

My first thought was that you’ve got more wind. Use a Chrony to rule out MV variability. Ambient temperature climbing or ammo in the sun. How long the round is in the chamber before firing.

Not more than 30 seconds.

 

[Brians356]

I've lived in NV over 50 years. One thing we get a lot of is mirage, as nearly everywhere load testing is performed is largely over bare ground, either decomposed granite or sand. At higher altitude with typically clear skies the solar is more intense. Ask any local dermatologist, if you can get five minutes of his time. They're very busy here!
-

 

[CharlieNC]

Hotter and higher means the effect of velocity differences on poi are different ie the moa drop/fps velocity between rounds. Probably higher velocity too. If you develop a load based on positive compensation, you have found the barrel exit time where the barrel harmonic slope offsets that ballistic slope. A change in ambient conditions can alter the ballistic slope sufficiently such that it is no longer in tune, thus requiring a load or tuner adjustment. If no "problem" is found tweak the charge weight, like the BR shooters during a match.