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Extreme Velocity Spread Question - I'm perplexed

 
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Old 06-26-2011, 12:04 PM
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Extreme Velocity Spread Question - I'm perplexed

I just got back from the range and am perplexed on what just happened. I have been working hard on my extreme velocity spread on my .338/.300 RUM/50 degree shoulder wild-cat.

2 weekends ago, i did some load testing and found a powder charge and seating depth combination that shot .5" groups (100 yards) with a 19 FPS extreme velocity spread.

I loaded up 50 rounds of this exact combination combination and headed to the range to zero the rifle and get an average FPS reading for my ballistic program prior to heading off to a long-range shooting school.

I shot 13 rounds and got a 64 FPS Extreme Velocity spread on the same load i shot 2 weekends ago with a 19 FPS spread (based only on 5 shots however)

NOTE: 19 FPS 5 shots were loaded with a dry neck lube and the 13 rounds fired with 64 PFS spread were not.

Any thoughts as to what might be happening here?

Below is some key data points that may help you in the quest of solve the problem or at least identify where the problem may be.

The only real difference is that the dry neck lube was NOT used on these loads.

Chronograph Data from this morning's shooting. Sorted from slowest to fastest.

2,772
2,792
2,797
2,798
2,808
2,809
2,809
2,810
2,812
2,815
2,820
2,836

Note: if you throw out the 2,772 and 2,836 shots the spread drops to 28 FPS which is a lot better but not single digits or low teens that i was expecting.

Question: Does Dry Neck Lube make that much of a difference???




Additional Information:
- PV-21 Chronograph was used.
- .300 Grain Verger VLD (Generation 2) in .338 caliber
- Bearing surface of all bullets was the same (measured with Davidson comparators x 2)
- Bullets were weighed on a digital scale and all were the exact same weight 300.4 grains
- All bullets were seated .010 longer than needed and then after each bullet was seated, i used the micrometer seating die to adjust each round down to the reading of 3.188 which in my rifle is .015 off the OGIVE.
- 93.0g of H1000 Powered (each load was exact loads)]All brass was trimmed to length, chamfered and de burred
- .363 Neck sizing busing was used on all brass
- Temperature was 75 degrees at the start of the shooting - 80 degrees at the end of the shooting
- All shooting was done in less than 1 hour time frame.
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  #2  
Old 06-26-2011, 01:05 PM
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Re: Extreme Velocity Spread Question - I'm perplexed

I feel it does make some difference. I haven't used it enough to say for certain but I had a similar experience. I did development w/o it got a load with an es of 12 on 2 three shot strings and one six shot string. It also shot under .5.. Put some of dry lube crap on and that made it shoot about 1". I didn't chrono it but I'll be I wouldn't be impressed. What we both probably should have done is to use the lube during development, rather than try it after we worked up.
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Old 06-26-2011, 01:06 PM
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Re: Extreme Velocity Spread Question - I'm perplexed

No answers here only a question. Was the sunlight consistant on the chrono?

Steve
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Old 06-26-2011, 01:59 PM
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Re: Extreme Velocity Spread Question - I'm perplexed

I can't think of anyway neck lube would have an affect.
What difference was there in brass size from load development to validation loads?
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Old 06-26-2011, 02:02 PM
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Location: Alaska
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Re: Extreme Velocity Spread Question - I'm perplexed

It sounds like you're being very careful and methodical with your load development, so I suspect you already know what your next step is. Dry neck lube another 5 rounds and see what kind of ES you get. If it matches the first 5 shots, then you'll know what the story is. If it doesn't match, then it's time for more load development - or another chronograph.

With only one chronograph, it's impossible to know if you're getting credible data or not, because you've got nothing to compare to. I run three chronographs concurrently to reduce the odds of being mislead with data from a single chronograph that might be acting up. The odds that more than one will spit out bad data on any single shot fired aren't very high. At the minimum, I can qualify the credibility of any velocity data obtained when the three chronographs aren't in close agreement.

Let us know what you conclude if you're able to figure it out.
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Old 06-26-2011, 02:03 PM
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Re: Extreme Velocity Spread Question - I'm perplexed

Quote:
Originally Posted by RockyMtnMT View Post
No answers here only a question. Was the sunlight consistant on the chrono?

Steve
No answers from me either, just a few questions...

what was rate of fire for each session? it's possible that a heated barrel would go 'faster'

where did the velocities from your first tests fall in the second session? (low end, middle, high end?)

In the end, the odds are your first session was not a sufficient number of data points to truly make a determination about the true ES of the load.
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Old 06-26-2011, 02:18 PM
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Re: Extreme Velocity Spread Question - I'm perplexed

Quote:
Originally Posted by RockyMtnMT View Post
No answers here only a question. Was the sunlight consistant on the chrono?

Steve
The PV-21 Chrono works off of infrared light and can be shot in bright sunlight or in the dark. Haven't tried the dark yet. it is suppose to be one of the highest quality Chrono's out there.
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