Velocity difference - My scales or chronograph???

[farmer7]

I've moved over to N140 from Varget for my .308 as I can no longer get it.

 

This week on Wednesday I was working up loads and settled on 45.3 grs N140 and averaged 2730 fps. Was a very sunny day and 20ºc. These were loads were measured on my RCBS 505 scales.

 

Yesterday I loaded up a large batch also at 45.3 but charged using my RCBS chargemaster. I then went out and shot some over the chrono and averaged 2706 fps. Yesterday was 16ºc but a very dull day.

 

Surely 4ºc couldn't affect my load by 24 fps? Therefore I'm thinking of blaming my Chargemaster or my chrono (Chrony F1).

 

I know the chrono is cheap but I've used it for years and always seemed to find it reasonably consistent. I would love a Magnetospeed but funds dont allow just now!

 

Any thoughts anyone? Could the sunny conditions versus the dull make any sort off difference?

[brown dog]

I've moved over to N140 from Varget for my .308 as I can no longer get it.

 

This week on Wednesday I was working up loads and settled on 45.3 grs N140 and averaged 2730 fps. Was a very sunny day and 20ºc. These were loads were measured on my RCBS 505 scales.

 

Yesterday I loaded up a large batch also at 45.3 but charged using my RCBS chargemaster. I then went out and shot some over the chrono and averaged 2706 fps. Yesterday was 16ºc but a very dull day.

 

Surely 4ºc couldn't affect my load by 24 fps? Therefore I'm thinking of blaming my Chargemaster or my chrono (Chrony F1).

 

I know the chrono is cheap but I've used it for years and always seemed to find it reasonably consistent. I would love a Magnetospeed but funds dont allow just now!

 

Any thoughts anyone? Could the sunny conditions versus the dull make any sort off difference?

 

Rule of thumb for 308 is 2.5fps per deg C - so 10fps of the drop is your reduced charge temp.

 

So, of the remaining 14fps ---well, for a non lab condition measurement, I'd say that's pretty stunningly accurate measurement.

 

(I'd caveat that by asking whether your sunny 20degC ammo had actually been sun-warmed to a higher temp than ambient - that would have skewed it to a higher mv).

 

Sun affecting mv - only by ammo/charge heating. You will find sun vs dull vertically affects your POI due to refraction in the optics, but that's not MV related.

[Jagged 77]

My opinion is that cheap chrono's are just that - how are they calibrated and how consistent are they really used in a non controlled environment? For me results on target are by far the most important thing. Forget velocity and concentrate on hitting the target.

[Wildcat69]

I have moved away from electronic scales as a primary scale, now back on beam scale although I double check my weights on a electronic as you never know. The results are much better with an es of 6-8fps in average on 3 shots string with my best load. I never managed to get this consistency before on an electronic one, unless i get an acculab. The brass is prepped in the same manner, neck turned, bushing die, fl sized, cartridge length and annealed. i don't think temp difference of only 4C0 would affect this much with N140. Beam scale are not always the most consistent but by double weighing can solve a major hurdle on load consistency.

 

Maybe worth trying to double check the same way as i do, at least you can identify the issue. Also chrony is good when it WORKS but I changed recently to a magneto one and it's absolutely awesome. (really major difference). Chrony don't seemed to work very well in blue sunny sky! Bad experience!!!!I I intend to give it away mine or shoot it!!!!!

[farmer7]

Rule of thumb for 308 is 2.5fps per deg C - so 10fps of the drop is your reduced charge temp.

 

So, of the remaining 14fps ---well, for a non lab condition measurement, I'd say that's pretty stunningly accurate measurement.

 

(I'd caveat that by asking whether your sunny 20degC ammo had actually been sun-warmed to a higher temp than ambient - that would have skewed it to a higher mv).

 

Sun affecting mv - only by ammo/charge heating. You will find sun vs dull vertically affects your POI due to refraction in the optics, but that's not MV related.

Thanks for the info. I was lying in the shade provided by the land rover whilst shooting but previously the ammo in an MTM box was inside the land rover for perhaps 10 minutes where it was fairly warm. The box was closed also. Maybe that would account for a few more fps??!

[farmer7]

Maybe worth trying to double check the same way as i do, at least you can identify the issue. Also chrony is good when it WORKS but I changed recently to a magneto one and it's absolutely awesome. (really major difference). Chrony don't seemed to work very well in blue sunny sky! Bad experience!!!!I I intend to give it away mine or shoot it!!!!!

I'll try weighing on both to check consistency.

 

Thanks for the info chaps!

[Andrew]

I've moved over to N140 from Varget for my .308 as I can no longer get it.

 

This week on Wednesday I was working up loads and settled on 45.3 grs N140 and averaged 2730 fps. Was a very sunny day and 20ºc. These were loads were measured on my RCBS 505 scales.

 

Yesterday I loaded up a large batch also at 45.3 but charged using my RCBS chargemaster. I then went out and shot some over the chrono and averaged 2706 fps. Yesterday was 16ºc but a very dull day.

 

Surely 4ºc couldn't affect my load by 24 fps? Therefore I'm thinking of blaming my Chargemaster or my chrono (Chrony F1).

 

I know the chrono is cheap but I've used it for years and always seemed to find it reasonably consistent. I would love a Magnetospeed but funds dont allow just now!

 

Any thoughts anyone? Could the sunny conditions versus the dull make any sort off difference?

You don't mention the sample sizes, the extreme spread, and standard deviation of your load. It may well be that the variance lies within the statistical probability of the load.~Andrew