Temperature Sensitivity Effects(or What it Affects?)

[Mr Underbridge]

Hi folks I have another question for the observant folks over here.
I was doing some research on reloder 17, but the powder is not really important.
I was wondering what the temperature changes affect?
Lets say I develop a load in a nice Rust Belt winter and I have found a good node for my rifle low ES and SD numbers and tiny groups.
If I shot that load when it was 100 degrees out and my velocities go up.
So the temperatures go up and the velocity goes up, can that shift my node and affect my SD and ES? Accuracy? Is it different compared to having a node shifted at the same temperature with a different amount of powder?

I use a cold bore ballistic program and I am pretty good about logging my temperatures and velocities, will I be able to compensate with a good ballistic program? Or if the swing in velocity is large enough it can screw up my groups and ES and SD numbers?

 

[gunsandgunsmithing]

In short, yes, it certainly does affect accuracy by simply changing where the muzzle is when the bullet reaches it. A tuner will fix it though!

 

[Ned Ludd]

The answer to your question depends on how much the average pressure/velocity changes between the two seasons [temperatures]. If it changes by an amount that would have put you out of the node/window even at the time you actually developed the load, it's probably going to be a lot further out in the warmer temps. There is no certain method of predicting what it would do to your ES/SD values. The safest bet is that they would increase as well, and the temperature-sensitivity of the specific powder you're using will play an big role in the magnitude of any changes. FWIW - RL17 is not a good choice in terms of temperature-sensitivity.

If you adjust the charge weight of your powder of choice such that the load gives the same velocity at the higher temperature (summer) as it gave during the colder months, it ought to put you pretty close. Reloading programs such as Quickload can generate estimates of how much the charge weight would need to be adjusted solely based on temperature. How good those estimates are will also depend on the powder, etc.

One of the problems in dealing with the effect you're describing is that the response to changing temperature (or charge weight for that matter) is not perfectly linear. Further, you're looking at least two effects: changing velocity and changing acceleration. For example, it is possible to develop loads with two powders having different burn rates to give the exact same muzzle velocity. When this happens, you know that the acceleration of the bullet in the barrel was not the same for the two powders. Barrel harmonics is dependent on what the bullet is doing as it traverses the bore, not what it does after leaving the bore (internal versus external ballistics). So how fast the bullet is moving as it exits the bore is only a small part of the equation. Powder burn rate, pressure increase rate, and acceleration while it's still in the bore are all variables that will affect its velocity at it exits the bore. Each of these variables can change when you A) change Lots of powder, B) change brands of powder C) change ambient temperature enough that your powder of choice behaves like a different powder.

So we're back to the initial caveat, which is exactly how much pressure and velocity change will you experience with a given powder/load. If it's noticeable, but not huge, simply adjusting charge weight lower to give same velocity will probably work out pretty well. If you're using a highly temperature-sensitive powder, it might require major tweaking or re-working to regain the same results.

 

[K22]

The most affect I've experience relative to temperature changes on Point of Impact and pressure is with ball powders such as H380 and H335.
Since I hunt both in the heat of the summer (ghogs) and cold in the winter (predators) with the same rifles / loads this is an issue for me.

Several years ago I switched from ball powders to Hogdon stick powders, namely H4895 and Varget. This minimized the effect and I didn't get any temperature surges like I did with H380 in the 22 250 in the summer. Also my POI change was less pronounced but I still had to adjust for changes in the increased air density in the winter in sub freezing temps, about 3/4" of an moa in elevation.

 

[mikecr]

Temperature affects more than powder burn. It also affects primers, striking of them, and attributes of a barrel.
If needing cold bore accuracy for hunting, it's best to work up a hunting load under season conditions.

 

[xswanted]

The answer to your question depends on how much the average pressure/velocity changes between the two seasons [temperatures]. If it changes by an amount that would have put you out of the node/window even at the time you actually developed the load, it's probably going to be a lot further out in the warmer temps. There is no certain method of predicting what it would do to your ES/SD values. The safest bet is that they would increase as well, and the temperature-sensitivity of the specific powder you're using will play an big role in the magnitude of any changes. FWIW - RL17 is not a good choice in terms of temperature-sensitivity.

If you adjust the charge weight of your powder of choice such that the load gives the same velocity at the higher temperature (summer) as it gave during the colder months, it ought to put you pretty close. Reloading programs such as Quickload can generate estimates of how much the charge weight would need to be adjusted solely based on temperature. How good those estimates are will also depend on the powder, etc.

One of the problems in dealing with the effect you're describing is that the response to changing temperature (or charge weight for that matter) is not perfectly linear. Further, you're looking at least two effects: changing velocity and changing acceleration. For example, it is possible to develop loads with two powders having different burn rates to give the exact same muzzle velocity. When this happens, you know that the acceleration of the bullet in the barrel was not the same for the two powders. Barrel harmonics is dependent on what the bullet is doing as it traverses the bore, not what it does after leaving the bore (internal versus external ballistics). So how fast the bullet is moving as it exits the bore is only a small part of the equation. Powder burn rate, pressure increase rate, and acceleration while it's still in the bore are all variables that will affect its velocity at it exits the bore. Each of these variables can change when you A) change Lots of powder, B) change brands of powder C) change ambient temperature enough that your powder of choice behaves like a different powder.

So we're back to the initial caveat, which is exactly how much pressure and velocity change will you experience with a given powder/load. If it's noticeable, but not huge, simply adjusting charge weight lower to give same velocity will probably work out pretty well. If you're using a highly temperature-sensitive powder, it might require major tweaking or re-working to regain the same results.

Have you physically tested R17 in extreme temperatures personally?

 

[Kings X]

In short, yes, it certainly does affect accuracy by simply changing where the muzzle is when the bullet reaches it. A tuner will fix it though!

So you can adjust your tuner to match the temperature? Is this sometime I must chart for each barrel ?

 

[gunsandgunsmithing]

So you can adjust your tuner to match the temperature? Is this sometime I must chart for each barrel ?

Yes, absolutely. Charts are great but sighters and the target trump charts. Tuners are so simple to use and adjust, I stopped charting everything several years ago and simply tune based on sighters. More often than not, I tune during the warm up and usually don't adjust again during the day..but it's there for those times when you do need to re-tune due to conditions.

Using a tuner is a good subject but is not really on topic. I'd be happy to discuss them. Phone is best. I've written a fair amount on these forums on the subject.

 

[Kings X]

I shoot F Class. Sometimes times we only get two sighters. I would want the tuner set before we start. That is why need to chart many different temperatures.

 

[gunsandgunsmithing]

I shoot F Class. Sometimes times we only get two sighters. I would want the tuner set before we start. That is why need to chart many different temperatures.

Understood...and yes, when you either can't see your sighters, or don't have the luxury of them..charting is very worthwhile. No different than charting for powder charge and seating depth, but it'll be less tune sensitive with the tuner than without.