What would cause the velocity to go up at lower temperatures?

T

Tbkahuna

Guest
I am working on the load for my Savage 12 FTR in .308.
I am in Houston and even though I shoot in the mornings mostly, the temperature is usually 80 degrees or higher, and by noon, it will be in the 90s or higher. I shot this morning when it was only about 55 or 60 degrees and warmed up to about 70s by the time I stopped.

My base comparison for Federal Gold Medal Match with 168 SMK is an average velocity of 2889 fps normally at 80s and warmer. This morning, in the cool, it was 2941 fps for a five shot string, and 2978 for a 9 shot string. It was from the same lot, and a box I had already partially shot. Two shots clocked over 3000 fps for the factory ammo in the 30" barrel Savage!!

In addition, I shot 42.5 gr. of Varget at-the-lands and averaged 2757 fps (that was fired brass) this morning. This is twice fired now, with only neck sizing with a Redding S bushing neck die.

Previously, once fired for 42.0 gr ATL was 2678, while 43.5 gr ATL was 2763. I was shooting 42.5 gr. this morning as I wanted to get the velocity up to 2700 fps from 2678. I get a little over 30 fps increase per .5 grain in warm weather based on all of my previous loads. However, in the cooler weather, my .5 grain raised the velocity from 2678 to 2757 which is 79 fps increase, and the 42.5 gr. was only 5 fps shy of what I got with 43.5 gr in warmer weather.

All reloads are with CCI BR2 primers, Lapua Brass, neck sized only, and Varget powder.

I didn't have any hard to extract cases, the brass looked fine, and the groups were good for not holding for wind.

Does anyone have any ideas on what could be causing this?

Rob in Houston
 
Maybe the suns angle on the chrony sensors? light can play tricks on your chrony at times. This might have been one of them.....:confused:
 
The powder is temperature sensitive.

most powders in cool temperatures exhibit decreased pressure and thus lower velocities. Tempurature inverse powders such as h4895 41.4 gr decrease in pressure as tempurature rises thus provide slower velocites in warmer tempuratures. Alliant Reloader 15 Is at max psi or cup at 70deg Farenheit above or below this temp and pressure eg velocity declines. Used for military loads in extreme warm or cold. Even the Hodgdon centralite coted extruded powders exhibit tempurature sensitivity.

Pressure also increases at a downward angle due to powder shifting forward in the cartridge alters its ignition characteristics.
 
Pressure increase @ low temp

TB -

This will be quick, before the rubber gun guys get here....


My 6mm wildcat shoots it's initial fouling shot ( or two ) high @ 100yd,
when I aim it at paper. It has always done this.
1/8" high and left, when it' s warm out. More... when it's cold(er) out.
IN PARTICULAR.... when OATs are low ( lower than 77*F ).

When the barrel is " cold ", the bore should be a skosh tighter.
Shoving a bullet down a tight bore, appears to cause chamber pressure to rise

I' ve seen this over and over with my rifle/barrel/ammo.

My fouling shots are taken now, not so much to " dry-out" any residual barrel conditioner ( that was applied after bore cleaning ); as they are taken to impart some warm-up activity to the bore.

I forgot to take my allowed single fouler shots BOTH days ( AAACKKK !!! )
of my attendance @ the West Pennsylvania Groundhog and Egg Shooting
Championships. Both days, my initial first " record shots "' went high left.
OAT was in the high 50s one day, it was also was quite damp that morning.

After the first record shot, the subsequent bullet holes all were found lower on the bull, SOME even where I had intended they'd show up !


Now, IF you' ll excuse me.... it's time for my medication.


Regards,
.357Mag
 
My powder was cooler.

I took it from the house at about 72 degrees and directly to the range. It was in a loading box for 30 minutes before the first shots and a couple of hours for the last. That is my typical pattern whether it is hot or cold outside. I never would have expected that much change.

I will recalibrate my scale, but it has been right on everytime I check it. I can compare new rounds from the same brass with the few I have left from the last batch.

The chrony gave ES and SD that were similar compared to other times, so I am thinking that wasn't it, but at this point anything seems possible.
 
The air temperature does not affect velocities. It's the temperature of the powder that is important. Of course, the temperature of the powder is affected by the air temperature and/or the temperature of the chamber and by how long it is exposed to either of those.

So, go back and re-analyze everything, taking into consideration how the temperature of the powder was affected. You might try some tests keeping your ammo at a constant temperature in a cooler, and see if the results differ.

Ray
 
I was just pointing out the time the ammo was in the various temperatures at the two different conditions.

The difference was on the cool morning it wouldn't have gotten any warmer than what it was in the house since it was cooler outside, but on the warmer days it would be warming up from that temperature. After several hours it would be close to the ambient temperature.

I am still analyzing every step.

Thanks.
 
Any change in the amount of time the cartridge sat in that warm chamber?
 
Not really. I ruined a .086" group with one that sat in the chamber too long and went a smidge high. I try to limit the time in the chamber to less than ten seconds at which point I fire it as a sighter, or eject it and put it back in the box to shoot later, once it cools down. They are never too awfully warm, especially when compared to normal weather in Houston.

What is stumping me is that even the factory Federal Gold Medal Match with the same bullets was much faster. I don't even know what powder is in those, but it was faster on that cool morning, also.

I looked in the chamber (no scope, though) to see if anything was obvious in there. Nothing I could see.

I had checked the cases for OAL and they were less than 2.010" with 2.006 being about the average, so I didn't trim any.
 
DaRealViper said:
The powder is temperature sensitive.

most powders in cool temperatures exhibit decreased pressure and thus lower velocities. Tempurature inverse powders such as h4895 41.4 gr decrease in pressure as tempurature rises thus provide slower velocites in warmer tempuratures. Alliant Reloader 15 Is at max psi or cup at 70deg Farenheit above or below this temp and pressure eg velocity declines. Used for military loads in extreme warm or cold. Even the Hodgdon centralite coted extruded powders exhibit tempurature sensitivity.

Pressure also increases at a downward angle due to powder shifting forward in the cartridge alters its ignition characteristics.
Click to expand...

Only problem is that H4895 is made with the same chemical technology and by the same company as Varget so they both would have very similar temp. sensitivity . Which is usually low.
 
Tbkahuna,

I live down south also. You know, where the air is almost always "heavy". The cooler weather you and I are experiencing is a lot drier than it normally is. The humidity was probably a lot lower when you fired those rounds than in the past. This would explain why both your reloads and the factory rounds chronied faster.

Just my .02 cents worth.
 
Yote,

According to the limited ballistic data I have read & the tables I have used, velocities are higher in humid air than dry air.

John
 
J. Valentine said:
Only problem is that H4895 is made with the same chemical technology and by the same company as Varget so they both would have very similar temp. sensitivity . Which is usually low.
Click to expand...

I have a phamplet from Hodgdon powder I would estimate from 2002 provides graphs and Data on the Extreme powders. Varget doesn't vary much from hot to cold pressure or velocity wise. H4198 and H4227 are temperature sensitive.

Hodgdon powders are mfg by ADI Powder co.

http://www.adi-limited.com/handloaders-guide/powder.asp

They use ethyl centralite coatings on extruded powder to reduce tempurature variations in both pressure and velocity; and to control burn rate of the powder.

Other companies use DNT di-nitro-tolulene to coat the nitrocellulose grains and ultimately vary kernel size to control burn rate. Dnt is much more sensitive to chg in temp than Centralite.

Also Single base vs double base extruded powders double base have an extra coating of nitro glycerin besides that contained in the nitrocellulose grains.
Higher throat temp and btu leads to reduced throat life with double base powders. Vihta Vouri and others add friction reducers (graphite) to their powder grains to increase barrel life.



H4895 example is Cartridge 22-250rem Case W-W Primer WLR Bullet 55gr Speed FMJ-BT I think the speed is a typo that should be speer.

Pressure 70deg F 49500 cup

Pressure 125deg F 48700 cup

Pressure 0deg F 49700 cup

Velocity70deg F 3742f/s

Velocity 125deg F 3748f/s

Velocity 0deg F 3728f/s

Pressure Variation (Hot) -800cup

Pressure Variation (Cold) +200cup

Velocity Variation (Hot) +6fps

Velocity Variation (Cold) -14fps

Extreme Pressure Spread 1000cup

Extreme Velocity Spread 20f/s

H4895 is temperature inverse powder as Deg F rises pressure (cup) decreases
and velocity increases.

The Reloader 12 from Alliant mfg by Bofors of Sweden no longer mfg
lists a 26f/s velocity increase along with 200(Cup) pressure increase from cold temperatures (eg 70 Deg vs 0 Deg F)

Normal Powder pressure and velocity increases with temperature. eg Win748

Varget has much lower variation hot to cold.
ES 8f/s velocity ES 400CUP Pressure.

Reloader 15 Powder reaches maximum psi at 70 deg F temps above or below
this and psi is less. Military Sniper powder for .308 long range loads armory contracts because above or below 70deg pressure of load is less than maximum. Summer Winter Siberia to Saudi Arabia.
 
That seems fair enough to me
 
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John Kielly said:
Yote,

According to the limited ballistic data I have read & the tables I have used, velocities are higher in humid air than dry air.

John
Click to expand...

John, I would love a source on that. Down in our area the humidity is very high. On a hot and humid day it takes effort just to breath. We aren't much above sea level. This increased humidity creates a drag on a bullet in flight thus slowing it down. I've noticed it on several occasions on my chrony.

If you could provide a source on that info, I would love to read it.
 
Yote,

I couldn't find a source that either of us would consider other than anecdotal, but I went aboard the JBM ballistic calculator on line & ran velocities for zero & 100 % humidity for the standard load they have on the template. The 100 % gave a few feet more remaining velocity (7 fps) at 1000 yards.

http://www.eskimo.com/~jbm/calculations/traj/traj.html

Interestingly, I struck one site that agreed with your view, so who knows?

John
 
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