Varying Adjustments at range?!!!

[farmerj]

I have shooting logs that say his software then is wrong.


M14 and M4A1 logs shooting from 6 am to 4 pm that show difference of 1-2 moa from early morning to mid-day sun.


Here's and interesting experiment for you to try....

Place 3 rounds in a cooler with ice.
Place three round on your dash of your car at the house.


Drive to the range and shoot two 3-round groups.

One with the hot ammo, the other with the cool ammo.


Let us know how it goes.

[andrewP]

I don't doubt your experience one bit. My guess is that the discrepancy has a lot to do with powder choice. My understanding (admittedly from loading handgun only, I've never reloaded rifle cartridges) is that some powders are much more temperature sensitive than others, which could easily lead to one having a much larger velocity spread from cold --> hot than another, which could then lead to a significant difference in how much of an adjustment is required when shooting in cold conditions vs hot. If the software's inputs were one powder vs your logs with another, well, they wouldn't be directly comparable at all, would they?

[jshuberg]

I have shooting logs that say his software then is wrong.


M14 and M4A1 logs shooting from 6 am to 4 pm that show difference of 1-2 moa from early morning to mid-day sun.


Here's and interesting experiment for you to try....

Place 3 rounds in a cooler with ice.
Place three round on your dash of your car at the house.


Drive to the range and shoot two 3-round groups.

One with the hot ammo, the other with the cool ammo.


Let us know how it goes.

You saw a 1-2 MOA difference at what distance? Air temperature changes effect external ballistics, meaning it changes the ballistic arc. Up close, there is very little if any difference, but the further out you go the slower the bullet will have it's velocity reduced by air, and will drop more. For example, with my load in my rifle the difference in 20* is .1" at 200 yards, but over 11" at 1000 yards. Depending on distance, a 20* temperature range can effect bullet drop anywhere from .05 to 1.1 MOA.

The temperature of a cartridge (as opposed to air temperature) affects internal rather than external ballistics. The warmer the powder, the faster the burn rate, and the higher the muzzle velocity. However, a 20* change in powder temperature isn't going to likely effect MV substantially enough to be appreciable at 200 yards. Out to 1000 and beyond, absolutely, but at 200 yards you would need a significant temperature change to be appreciable. For example, using my load and .308 rifle, a 50fps difference in MV corresponds to only 0.2" at 200 yards.

Here is a link to an online ballistic calculator:
http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

Please show how any reasonable setup could result in a 3" difference at 200 yards from a 20* temperature difference.

The science here is well known. If anyone is blaming a 20* temperature difference for a 1-2MOA POI shift at 200 yards, they don't understand the science, and have been guessing the wrong variable as being the culprit. Most likely, altitude-density (station barometric pressure) needs to have changed significantly in order for those kinds of differences to be observed that close, not temperature.

[jshuberg]

My understanding (admittedly from loading handgun only, I've never reloaded rifle cartridges) is that some powders are much more temperature sensitive than others, which could easily lead to one having a much larger velocity spread from cold --> hot than another, which could then lead to a significant difference in how much of an adjustment is required when shooting in cold conditions vs hot.

That's actually not the case. Muzzle velocity changes due to powder burn rate due to cartridge temperature are pretty much the same across powders and applications. All of the conversion charts I've seen have been based off Army Howitzer tables, and have proven to be extremely accurate in predicting MV variations for small bore cartridges out past a mile before they need tweaking. The reason is that regardless of the combustible material, there is a consistent and predictable increase in it's combustibility rate as temperature changes. What most people mean when they say "temperature sensitive" powder is the flash point of the powder, not that it has a non-standard combustibility increase with regard to temperature changes.

[andrewP]

Muzzle velocity changes due to powder burn rate due to cartridge temperature are pretty much the same across powders and applications. All of the conversion charts I've seen have been based off Army Howitzer tables, and have proven to be extremely accurate in predicting MV variations for small bore cartridges out past a mile before they need tweaking. The reason is that regardless of the combustible material, there is a consistent and predictable increase in it's combustibility rate as temperature changes. What most people mean when they say "temperature sensitive" powder is the flash point of the powder, not that it has a non-standard combustibility increase with regard to temperature changes.

If that's always the case, what's the explanation for some powders being reverse temperature sensitive (higher velocities observed at the chrono when colder)? I don't have any examples off the top of my head, but I've definitely heard people talk about that before. Edit: I just found someone talking about WST loads gaining velocity in colder weather.

Just for the record, I'm not trying to be a pain in the rear; I'm just curious why what you're saying seems to run counter to what I've seen discussed in the past, and trying to learn.

[jshuberg]

The more powder in a cartridge, the more a faster burn rate will increase pressure before the bullet leaves the barrel, MV changes due to temp do depend on the weight of the powder charge, but I've never seen anything actually measured where two different powders of the same weight will have their MV change at different rates as temp increases. Not saying it can't happen, but all the burn rate conversion charts I've seen are powder agnostic.

As far as lower temps producing higher MV's, again I can't say that it's not possible, but I don't think it has to do with burn rate. It defies the laws of physics that two substances that are exactly the same except for temperature will result in the colder substance burning faster. If someone is experiencing higher MV's when it's colder out, I'd tend to point my finger at metallurgical changes, or perhaps a temperature sensitive/malfunctioning chronograph.

It's a very complicated subject. Lots of things come into play, but simply stating that a difference in POI from morning to afternoon shooting is the result of temperature, excluding all of the other possible environmental and mechanical conditions that are likely also taking place greatly oversimplify the problem. I've precisely input my load and rifle data into my ballistic calculator. When setting up I simply input station pressure, temperature, humidity, approximate wind conditions, guestimate my MV based on how recently I cleaned the barrel, and the calculator always gets me on target out to 1000 yards. If there was something wrong with the maths or my inputs, I doubt I'd be able to hit paper first shot.

[farmerj]

Have you actually run and used a range data book?

And then compared them to the computer output?

[jshuberg]

I keep a log of all my various MVs I've measured under various conditions, but I only shoot rifle recreationally, so I'm not religious about logging everything. Thats why I use a calculator, to keep it as easy as possible for myself. It's always got me on paper out to 1000, summer though winter under widely varying environmental conditions. Once I'm on paper, then it's just me on the gun trying to get my groups nice and tight. If I start drifting off center, I reenter the current environmentals and I'm back again. People a lot smarter than me have worked out all the complexities and maths, and I have no reason not to trust it. It simply works every time, under all conditions. I've been thinking about getting a whiz-wheel built for my rifle/load combination so I don't have to rely on a battery powered device.

https://www.youtube.com/watch?v=GIaUb-lYJC8

[20mm]

Did you take coriolis effect into account?

[farmerj]

Getting you on paper is one thing

Having no or maybe one Spotter shot, is an entirely different game.


You need to know your dope solid when making that cold bore count. That only comes from actual range time. And count can mean either a "x" ring or a sniper shot. Take your pick.

We ran books from 300 meters to 1000 meters.

Much like the NRA matches at the gopher range.

[jshuberg]

Super. Then tell me in detail exactly how a 20* difference in air temp translates to a 3" difference of a .308 round at 200 yards. Because the math says it's not possible unless your MV is ridiculously low.


Sent from my iPhone using Tapatalk

[farmerj]

I already posted it.

Place three rounds on your dash and three rounds in a cooler with ice.

Then go shoot two 3-round groups.

[jshuberg]

Your dash is typically over 100* in the sun. Ice is less than 32*. Thats close to a 70* difference in cartridge temperature, not a 20* difference in air temperature as you originally stated. Please try again. And provide actual measurements of bullet, load, rifle, and environmental conditions. I can give you actual measured numbers that you can plug into the ballistic calculator I linked to above, I expect that you can do the same from your log book entries. Lets look at the science of your claim.

Or maybe science is a lie of the devil, and you're just always right.

[farmerj]

We used m118LR 7.62 ball and M855 ball ammo.

You still claim that temperature will not effect zero.

It does.

As does humidity and if you have a tail or head wind.

As you said, your software gets you on the paper. That's it.

It doesn't dial you in for a first hit cold bore shot.

Hard data only acquired from actual range time provides that.

[jshuberg]

We used m118LR 7.62 ball and M855 ball ammo.

You still claim that temperature will not effect zero.

It does.

As does humidity and if you have a tail or head wind.

As you said, your software gets you on the paper. That's it.

It doesn't dial you in for a first hit cold bore shot.

Hard data only acquired from actual range time provides that.

Reread the post. I have absolutely stated that temp effects POI. At 200 yards, everything else being the same, a 175gr SMK bullet with a MV of 2603 will impact exactly 0.1" low with a 20* temperature drop. At 1000 yards it will impact just over 11" low. A lighter, faster bullet would be effected less. What I'm saying is that your claims of a 20* temp difference causing a 3" shift of POI at 200 yards is bull$hit. I have never once said that temperature doesn't effect zero.

And when I said that my ballistic calculator gets me on center paper at 1000 yards, that accounts for my shooting skills. It usually takes me several strings behind the gun to get myself dialed in. That is not in any way an inditement on the ability of a ballistic calculator to accurately predict POI, but on my long distance shooting skills, which unfortunately can be rather inconsistent. Science works. Math works. There are shooters that can hit a 12" steel target at 1 mile using nothing but a ballistic calculator, and by anticipating the current MV based on bore conditions, etc.

There simply is no way that a 20* change in air temp can result in a 3" POI shift at 200 yards. If that's what your books say, then your guess that it was temperature that caused it is wrong.

You gave me the bullet that you used. If you could also provide for me the muzzle velocity, twist rate, zero distance, sight height, station pressure, humidity, target angle, wind conditions, and the two different temperatures you were shooting at, we can put this to rest once and for all by applying the calculations.

Unless you can provide numbers that show otherwise, there's no reason to continue the discussion. Science says you're wrong.