Ambient Temp vs Varget load

[500Stroker]

Is there a rule of thumb for reduction of your powder (Varget) load as the ambient temp climbs in the hotter months?
I currently have a very good cold weather (30-40 degree F) load (31.7gr) for my 6BRX and I want to develop a hot weather load (80-90 degree F).
What is a good starting point?

 

[T-shooter]

I hope someone has some input on this. I went to Varget over IMR4064 because it seems too sensitive. Loads that shot accurately at 50-60 degrees didn't in the middle 80's to middle 90's. I thought about trying to test this. Put some rounds in the fridge all night and keep them in a cooler, loading one at a time. Directly compare with some that have acclimated to the outside temperature. There is another aspect. Hot and humid air is less dense and your velocities will go up regardless of the temp of the rounds. May be able to figure the approximate difference in velocity based on air temperature using a ballistics calculator and figure that into the readings.

 

[Tiratore]

There is data in the Vihta Vuori manual for their powders, but it's for powder temperature which is arguably the same most of the time.

 

[jpmimproved]

I hope someone has some input on this. I went to Varget over IMR4064 because it seems too sensitive. Loads that shot accurately at 50-60 degrees didn't in the middle 80's to middle 90's. I thought about trying to test this. Put some rounds in the fridge all night and keep them in a cooler, loading one at a time. Directly compare with some that have acclimated to the outside temperature. There is another aspect. Hot and humid air is less dense and your velocities will go up regardless of the temp of the rounds. May be able to figure the approximate difference in velocity based on air temperature using a ballistics calculator and figure that into the readings.

I know with my reloads with R15 are fine in 90 degrees, but as soon as my barrel heats up, I start getting pressure signs. Ambient temperature is a player, but you'd have to make sure the round didn't sit in a hot chamber too long.

 

[T-shooter]

I know with my reloads with R15 are fine in 90 degrees, but as soon as my barrel heats up, I start getting pressure signs. Ambient temperature is a player, but you'd have to make sure the round didn't sit in a hot chamber too long.

Is it because that the round sat in the chamber absorbing heat or the barrel itself changing dimensions. It seems I get them occasionally on the first cold shot. Things expand with heat. Does the bore get larger when heated enough to make a difference?

 

[ledward]

Is there a rule of thumb for reduction of your powder (Varget) load as the ambient temp climbs in the hotter months?
I currently have a very good cold weather (30-40 degree F) load (31.7gr) for my 6BRX and I want to develop a hot weather load (80-90 degree F).
What is a good starting point?

1 foot per second increase in velosity for every 1 deg F increase is a rough rule of thumb I use for Varget.
My loads that I use in the 20-30 deg range will increase 50fps when the temp is 70 degs. So to answer your question i would decrease half a grain to get you in the ball park.
This is in a dasher.

 

[500Stroker]

1 foot per second increase in velosity for every 1 deg F increase is a rough rule of thumb I use for Varget.
My loads that I use in the 20-30 deg range will increase 50fps when the temp is 70 degs. So to answer your question i would decrease half a grain to get you in the ball park.
This is in a dasher.

That's what I was look'n for - Thanks

 

[swd]

If you've chrono'ed the load at 30-40 deg just adjust to match that speed at 90 deg. But yes a half grain is in the ball park.

 

[Jet]

500Stroker

This is specific to Varget and most of the powders I use. To counter the effects of temperature, humidity, elevation, etc I find that selecting a powder charge in the middle of a tune window (ladder test) then adjust seating depth, neck tension, etc works when conditions change (rifle does not come out of tune). If an accuracy load is on the edge of a tune window it will likely come out of tune when conditions change and cause a lot of head scratching.

I adjust powder charges based on how much throat erosion is going on and where I am seating the bullets in the case over what the temps are doing.

Good Shooting

Rich

 

[Bart B.]

Is it because that the round sat in the chamber absorbing heat or the barrel itself changing dimensions.

Hot barrels heat powder.

Shooting mid and long range matches, I've always come down a click on the sight for every 30 seconds the round set in a 308's hot chamber. Every 20 seconds for 30 caliber magnums.

People have lost big long range matches at the NRA Nationals with winning score logged most way in their score sheet letting rounds get hot during a cease fire for boats in the impact area, then near 2 minutes later after Commence Fire was announced, that round put its bullet in the 8 ring at high noon. They finish with a 198 with lots of Xes placing 21st out of several hundred. Had they shot a 10, they would have still won the match.

The few millionths (ten thousandths?) of an inch bores expand from heat has no effect on accuracy. How else could someone shoot 40 shots from a 308 twenty seconds apart and they all land well inside 2 inches at 600 yards. I've put 30 inside 5.5 inches at 1000 starting with a cold barrel chambered for a 30 caliber magnum fired about the same rate testing two 15-round lots of ammo shot alternately.

 

[T-shooter]

Hot barrels heat powder.

Shooting mid and long range matches, I've always come down a click on the sight for every 30 seconds the round set in a 308's hot chamber. Every 20 seconds for 30 caliber magnums.

People have lost big long range matches at the NRA Nationals with winning score logged most way in their score sheet letting rounds get hot during a cease fire for boats in the impact area, then near 2 minutes later after Commence Fire was announced, that round put its bullet in the 8 ring at high noon. They finish with a 198 with lots of Xes placing 21st out of several hundred. Had they shot a 10, they would have still won the match.

The few millionths (ten thousandths?) of an inch bores expand from heat has no effect on accuracy. How else could someone shoot 40 shots from a 308 twenty seconds apart and they all land well inside 2 inches at 600 yards. I've put 30 inside 5.5 inches at 1000 starting with a cold barrel chambered for a 30 caliber magnum fired about the same rate testing two 15-round lots of ammo shot alternately.

I haven't fired my .308 to get it that hot and don't leave rounds in the chamber to heat soak, but my SKS will start stringing the shots out as the barrel heats up. 30 shots in 5.5" at 1,000 is some fine shooting!

 

[Bart B.]

If the barrel's fit to a receiver whose face is squared up well with the barrel tenon axis, there'll be no hard contact point around the tenon shoulder. As the barrel (and receiver) heat up and expand, it'll be contacting the receiver hardest at that point. Stress lines in the barrel bend it relative to that point. Shots start walking away from POI when barrel was cold. All barrels I first had and others I know of always walked shots in the same direction each time. All of mine did until I was advised to get stuff squared away. Two match rifles with good barrels quit walking shots as they heated up.

 

[varmtr]

Varget or known over here in Aus AR2208 both the same powder made by ADI.
AR2208 is very temp stable as you guys are well aware temps here in Aust can get bloody hot. I have yet to see or here guys here complaining about pressure issues using this powder due to ambient temps

 

[RDavies]

Varget or known over here in Aus AR2208 both the same powder made by ADI.
AR2208 is very temp stable as you guys are well aware temps here in Aust can get bloody hot. I have yet to see or here guys here complaining about pressure issues using this powder due to ambient temps

It is not the temperature which is the issue, it is the temperature CHANGE. Next time I shoot will be around the freezing point but last time I shot the Dasher was in warm weather which is what the load is tuned for. I usually add .2gn to my cool weather load but since it will be around freezing point I will likely add closer to .4gn. The 1fps drop per 1 degree F rule is about right for Varget (2208)

 

[Bart B.]

I've shot Varget in 308's pushing 155-gr. bullet out towards 1000 yard targets in ambient temperatures from the low 20's F to high 90's F. Hot and cold ammo had no significant change in accuracy.

Stool shooters add a click or two to charges to keep bullets leaving a bit faster to be stable in cold, thick air. I doubt the ammo's temperature will have any effect on accuracy. Tests will prove what's reality.

 

[K22]

The only powders I have experienced pressure surges in hot weather, 90+ are ball powders, H335 and H380. I used Varget for years in the 223 Rem, 243 Win, and 308 Win and have never had pressure surges. POI will change from extreme heat to cold, usually a few fractions of a minute but I beleive this is due to changes in air density rather than temperature.

 

[Bart B.]

The only powders I have experienced pressure surges in hot weather, 90+ are ball powders, H335 and H380

Another reason ball powders are not popular for producing best accuracy.

 

[K22]

Another reason ball powders are not popular for producing best accuracy.

On the contrary, these two powders are extremely accurate in the 223 Rem (H335) and the 22 250 (H380). But in my experience I have had pressure surges, especially with H330 in 90+ weather hunting ghogs. The problem was that I didn't adjust the load that was developed in the spring to hot weather conditions.

Because I didn't want to deal with having to adjust loads from cold to hot field conditions I switched to Hodgdon stick powders which are more temperature tolerant but these ball powders were very accurate (small groups) in my rifles.

 

[Bart B.]

On the contrary, these two powders are extremely accurate in the 223 Rem (H335) and the 22 250 (H380)

Are all your test groups under 3/10 MOA at 100 yards?

 

[K22]

Are all your test groups under 3/10 MOA at 100 yards?

No - I'm not that consistent to shoot to the full capability of some of my rifles that have match barrels. If I'm in the zone and the weather conditions are ideal my Weatherby and Rem 700 with a Douglas Match barrel will group (5 shots from a cool barrel) consistently in the 1/4 moa range at 100yds off the bench with front and back rests. Some of my other rifles are in the 1/2 moa category, some are in the 1 moa category.

Where are you going with this?