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Can someone explain the big difference in velocity?

 
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  #1  
Old 07-07-2010, 12:50 PM
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Can someone explain the big difference in velocity?

Well I thought I had my new load figured out, until today. Two weeks ago I worked up to 60 gr of 4831sc with the 180 Bergers, 5 shots varied only 4 fps, and averaged 2942 fps. I loaded several rounds exactly the same and hit the range this A.M. just to double check. My shots, in a fouled barrel, measured 2848, 2864, 2896, 2900, 2902 (after cooling the barrel for 13 minutes), 2896, and 2904. All the shots were fired at least 2 minutes apart. The weather today was 68 deg and sunny. Due to the bad mirage I couldn't tell which shots impacted where but all were within 2.5" and three were .58", all were shot at 300 yds. Any Ideas as to why the huge velocity difference?
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  #2  
Old 07-07-2010, 12:59 PM
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Re: Can someone explain the big difference in velocity?

What brand & model is the chronograph?
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  #3  
Old 07-07-2010, 03:32 PM
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Re: Can someone explain the big difference in velocity?

The first time all the stars were aligned - I know it is frustrating but you did what was right in checking something that you only wish for. Any rifle/reload combination will be dead on ONCE - to repeat it is the challenge. Good luck - and by the way - you are not alone on this - trust me.
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Old 07-07-2010, 04:08 PM
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Re: Can someone explain the big difference in velocity?

Was the chronny all the way in the shade? And was it set up at the same distance?how long were the bullets in the ''cooled chamber'' before you shot again? (first day vs 2nd day) depending on how long said ammo was in the chamber before you squeezed one off it can make a slight difference, but not that much I wouldnt think.
I always thought 4831 was extremely stable, so I doubt it would be that. Maybe a combination of all the issues above may give a slightly bigger variation than the usual e.s. (imo the usual being alot more than 4fps, but much less than the 64 fps)
Very strange that they avg 2942 the first time, and so much slower and such a large spread the second go round. 2848 (extrm low) How was the humidity when you loaded the first batch vs the second batch? Im just takin shots in the dark here. Very odd.
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Old 07-07-2010, 07:26 PM
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Re: Can someone explain the big difference in velocity?

Just guessin' here. As soon as you post which chrono you're using, the first comment may apply.

I figure you're shooting that & SAUM. Also Assuming a brake.

Move the chrono farther out. Maybe as much as 12 to 15 feet total.

Learned on this forum that on larger cases w/brake low ESs were tough to come by on a regular basis.

If the chrono is a Chrony and sunny means bright with small clouds and the clouds sometime shade or partially shade the chrony velocities will vary. Also watch for the shadow of the shade rods crossing the sensor. One sensor in the shade of a rod and the other not, gives me the hebeejeebees.

Also I 'think' I've learned that with the Chrony bright means 20-40 FPS higher indicated velocity over a hazy day or when a cloud shades the thing.

I cut a 2'x2' piece of 1 inch Styrofoam and shade the whole thing. Seems to help w/consistency.
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  #6  
Old 07-08-2010, 02:10 AM
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Re: Can someone explain the big difference in velocity?

Sorry about the lag in reply, work always gets in the way.

I have a Chrony Beta Master that I've had for about a year.

I should have mentioned that I was shooting my 7 SAUM with the CSR brake.

I had the chrony sitting at almost full length of the cable to the remote read-out, maybe around 10 feet.

From my notes, and memory, (mainly notes) both days were very similar in terms of weather. I was shooting from the same bench but to the east on the first day and north on the second. I didn't notice if any shadows were being cast from the metal arms, there's no trees for miles, but I'll try the cover idea though.

Thanks for the input.
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ACTIVE DUTY MARINE CORPS '96-'04
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"Be polite, be professional, but have a plan to kill everyone you meet."
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  #7  
Old 07-08-2010, 05:39 AM
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Re: Can someone explain the big difference in velocity?

What you experienced isn't uncommon. Heat = increased velocity. There have been times on the bench when I've used my watch to make the velocity of the next round come out to what I wanted. Wait a bit longer and the velocity comes down - shoot a bit quicker and the velocity walks up.

With a properly worked up load (like via a ladder test) you'll have a load where the temp will effect it substantially less than other loads.
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