Chronograph results

bkhann · July 16, 2010, 02:14 PM

I just re-chrono'd several loads that I developed this spring. In the spring the outside temperature was about 50 degrees, today the temperature is around 90 degrees. In the warmer temperature I am noticing about 5% slower bullets speeds.

Anyone else experience this?

totaldla · July 16, 2010, 03:49 PM

Seems backwards. Most powders achieve higher pressures, hence higher velocities on warmer days. I don't know of anything with a negative temp coefficient. I wonder if the Chrony expands enough to change the readings?

4runnerman · July 16, 2010, 04:56 PM

im with Totaldla on this one. Seems real strange. Did you use same powder, same case col, same primer,ect ect?. Was the crony in sunshine or anything. Warmer temps should have increased your speed or at least stayed the same. Some varaince can be lots of powder ,but very little change.

Brian Pfleuger · July 16, 2010, 05:00 PM

Are these max or near max loads?

Decreasing speeds can actually be a sign of an over-pressure load.

bkhann · July 16, 2010, 08:07 PM

These are not near max loads. Generally they are 9mm @ 1000-1100 fps. Sort of middle of the road.

One thing that is different is that in the spring I was taking readings at 10 feet, now I take all readings at 15 feet. 10 feet caused some errors with 357 magnum loads, so I decide to measure all loads at 15 feet.
I would not expect to see a 5% drop in fps but perhaps?

I will contact the chrony people on Monday to see if they have any thoughts.

Brian Pfleuger · July 16, 2010, 08:27 PM

10 feet is close enough that your readings may have been effected by muzzle blast, which is what you found with the 357. The original reading from 9mm could have been effected.

Are these all from the same bottle of powder?

Also, 5% of 1000 is only 50 fps. How many rounds are you talking about? 50 fps is miniscule. Such a variance is well within the realm of environmental variations. Temperature, humidity, air pressure.

A slight difference in seating depth or crimp could probably account for 50 fps, particularly when coupled with all the other variables.

Anyway, it is certainly not different enough that it would cause me any concern whatsoever.

WESHOOT2 · July 17, 2010, 05:58 AM

I regularly retest ammo stored for years; I test it in the heat of summer and in the rain and at subzero cold and whenever I think of it; I test it after storing it in the trunk of my car for eighteen months and sitting out in the garage and then I know.

That's what learning is about.

I don't argue with data; I just collect more......

July 16, 2010, 02:14 PM	#1
bkhann Member Join Date: August 5, 2009 Location: Grand Rapids MI Posts: 98	Chronograph results I just re-chrono'd several loads that I developed this spring. In the spring the outside temperature was about 50 degrees, today the temperature is around 90 degrees. In the warmer temperature I am noticing about 5% slower bullets speeds. Anyone else experience this?

July 16, 2010, 03:49 PM	#2
totaldla Senior Member Join Date: August 11, 2009 Location: SW Idaho Posts: 1,497	Seems backwards. Most powders achieve higher pressures, hence higher velocities on warmer days. I don't know of anything with a negative temp coefficient. I wonder if the Chrony expands enough to change the readings? __________________ Keltec P15 at 1200 rounds

July 16, 2010, 05:00 PM	#4
Brian Pfleuger Moderator Emeritus Join Date: June 25, 2008 Location: Austin, CO Posts: 19,694	Are these max or near max loads? Decreasing speeds can actually be a sign of an over-pressure load. __________________ https://ecommercearms.com I am the owner/operator! Ask me for custom prices! No sales tax outside CO!

July 16, 2010, 08:27 PM	#6
Brian Pfleuger Moderator Emeritus Join Date: June 25, 2008 Location: Austin, CO Posts: 19,694	10 feet is close enough that your readings may have been effected by muzzle blast, which is what you found with the 357. The original reading from 9mm could have been effected. Are these all from the same bottle of powder? Also, 5% of 1000 is only 50 fps. How many rounds are you talking about? 50 fps is miniscule. Such a variance is well within the realm of environmental variations. Temperature, humidity, air pressure. A slight difference in seating depth or crimp could probably account for 50 fps, particularly when coupled with all the other variables. Anyway, it is certainly not different enough that it would cause me any concern whatsoever. __________________ https://ecommercearms.com I am the owner/operator! Ask me for custom prices! No sales tax outside CO!

July 17, 2010, 05:58 AM	#7
WESHOOT2 Senior Member Join Date: February 20, 1999 Location: home on the range; Vermont (Caspian country) Posts: 14,324	learning I regularly retest ammo stored for years; I test it in the heat of summer and in the rain and at subzero cold and whenever I think of it; I test it after storing it in the trunk of my car for eighteen months and sitting out in the garage and then I know. That's what learning is about. I don't argue with data; I just collect more......

July 16, 2010, 04:56 PM	#3
4runnerman Senior Member Join Date: January 16, 2010 Location: Minnesota Posts: 3,577	im with Totaldla on this one. Seems real strange. Did you use same powder, same case col, same primer,ect ect?. Was the crony in sunshine or anything. Warmer temps should have increased your speed or at least stayed the same. Some varaince can be lots of powder ,but very little change.

July 16, 2010, 08:07 PM	#5
bkhann Member Join Date: August 5, 2009 Location: Grand Rapids MI Posts: 98	These are not near max loads. Generally they are 9mm @ 1000-1100 fps. Sort of middle of the road. One thing that is different is that in the spring I was taking readings at 10 feet, now I take all readings at 15 feet. 10 feet caused some errors with 357 magnum loads, so I decide to measure all loads at 15 feet. I would not expect to see a 5% drop in fps but perhaps? I will contact the chrony people on Monday to see if they have any thoughts.