Can someone explain the big difference in velocity?

[JARHEAD1371]

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?

 

[Pdvdh]

What brand & model is the chronograph?

 

[cowboy]

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.

 

[winmag]

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.

 

[royinidaho]

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.

 

[JARHEAD1371]

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.

 

[Miller Outdoors]

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.

 

[royinidaho]

I was shooting from the same bench but to the east on the first day and north on the second.

That will do it every time. The affect has been know to be called the "crapyollis factor". No known solution but Broz has been working on it.

Try to maintain a consistent moon phase when doing serious shooting.

 

[cfvickers]

I am dealing with the same thing with my 6.5-284. Accuracy is as good as ever, but velocity has gone down in the last month. So here are my thoughts on why MY problem exists. 3 weeks ago I was getting higher velocity with everything I tried, and very low SDs as well. My powders were H4831SC and reloader 17. I am getting 70 FPS less from the RL17 and 50 less from 4831. At the time I started working with the loads the average temp was 85 degrees during the day, now it is 97, Humidity was 70% now 85, and I believe most importantly, Barometric pressure was 29.80 now it is 30.08. Also I am getting pressure issues with the reloader 17. I think the pressure problems along with decreased velocity probably are due to the temp and barometric pressure. Supposedly with RL17 it is supposed to have a more uniform pressure curve but when you add variables there is no way to avoid changing that. In what way, I don't know, but it looks like I could be having a spike in pressure to dangerously high levels that is caused by my outside conditions. The reason I think this is that a pressure spike and quicker drop would considerably decrease velocity, and when working with maximum loads as I was it would cause major pressure signs. I will test my theory in a couple months when the temps and outside air pressures stat to go back down, until then it is just a theory but I got bullets to pull. With the H4831SC all I have noticed was decreased velocity but no pressure signs, on the other hand I am working with a moderate load 3 full grains under hodgdon's listed max. Now at 68 degrees, that sounds odd, I would theorize barometric pressure and humidity, but I don't know physics, just what I have read about the burning characteristics of various powders. I know the characteristics of Hybrid 100V and RL17 are supposed to be similar but the hybrid seems to be more consistent in these conditions here than the RL17, and less consistent in lower temps.

 

[royinidaho]

What if you used an ice bath and didn't let them set in the chamber very long?

There seems to be some concern regarding differences in humidity when reloading?

 

[coyotezapper]

Something has to be different?? Component change? Lot change within component? Checked/calibrated scale? Door closed or open while throwing powder charge, breeze/draft. Have you weighed your brass? Chrono set up the same, same spacing, sunny or shaded? Anything different in cleaning process such as bore treatments?

 

[Pdvdh]

I have a Chrony Beta Master...

There's one potential explanation.

 

[cfvickers]

I have a chrony beta, and and prochrono of some sort, its a basic one. Both read within 10 FPS of each other every time I have checked them.

 

[Pdvdh]

Set the two of them up so they both record every shot. Then over time you'll learn whether or not they perform consistently and reliably.