A reload spike in fps ?

T

thekskid

Guest
I have a new 6.5 creedmoor dpms ar rifle. I did some chrony testing today and one round came up on the chrony at 2950. The rest of the loads shot 2754 2738 2682 2688. My question is what would cause one round to shoot so much higher other than this one round having more powder in it.

I'm wondering if the the case bit the bullet or something else anyone have some food for thought for a young guy that could cause this jump in fps. this is my third run through the chrony moving up in a grain in the second reloads. The reloads moving up 1.5 grains shot one at 2950.

Thanks
 
alinwa said:
Are you individually weighing every charge?

al
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No sir

Harrell powder thrower, we set it weighed then charged the cases with the powder thrower. Per a friend who is more experienced than me "ie" someone on this forum.
 
what powder ???

typically its still plus or minus a tenth with very short n133.....so maybe your powder did not do so well in the thrower....
a small case, a mag primer, a bullet seated short

which shot in the string ??
 
I would question the accuracy of the chronograph. Sudden spikes like this are common outdoors, on the chrono that is, not the rifle. Also where did this "fast" shot strike in relation to the other shots on paper?
 
Had this happen Friday

Shooting right along @ 2945 or so and suddenly had two that went to 3660 something. No tight bolt or blown primers. I was shooting slow and the sun's angle right then was obtuse so I laid it to that. The other strings I shot were ok.

Interesting note on cronoing: A couple of friends were doing some Crono work the other day and had a round that went quite low in speed. One of them took that case and reloaded it thinking surely he had a "Bad Case". When he shot it again the speed went beyond where the others were recording. He asked me what I thought. I told him it might very well prove my theory which is, it's what one puts IN the case that matters a lot more than the case itself. These friend "Throw" charges.
 
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From what I've read about the 6.5 Creedmoor it uses fairly slow and large granuled powders which tend to not meter as well as faster and smaller granuled powders do. That much velocity change would indicate a fairly large increase in charge weight though. It could be due to powder bridging in the measure, but if the charged cases were inspected before bullet seating it should have been visible unless the powder was below the shoulder of the case.

Try weighing a set of charges and chronographing them to see what sort of results you obtain.

I've had similar things happen, but found it was due to an insect flying through the lightscreens at just the wrong time, or some other chronograph error.

An unnoticed high charge, or a problem with the chronograph are the most likely problems I'd think though.
 
No way the spike could be cause from reloading errors other than powder charge. Could neck tension or the neck bitting the bullet cause the spike.
 
Don't be too sure.

A primer seated less than optimally (differently to the rest) on a charge of slow powder might cause ignition issues leading to pressure increasing.

By the way, is there any chance the cases need length trimming?
 
John Kielly said:
Don't be too sure.

A primer seated less than optimally (differently to the rest) on a charge of slow powder might cause ignition issues leading to pressure increasing.

By the way, is there any chance the cases need length trimming?
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Hmm a friend trimmed the cases for me with his old case trimmer. He modified a pilot since he didn't have the correct size. I wonder if the case length being to long would have caused the problem. My only concern here besides accuracy is safety.

Thanks for all the advice everyone.
 
Thrown charges can and will easily vary 100fps.

A too-long case absolutely WILL cause pressure spikes. This happened on my first ever 6BR. I was told it was a tightneck but not shortened.... my first loads were too long in the neck by just a couple thou....

DAN'rous!

al
 
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