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Erratic Chronometer Readings

 
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  #1  
Old 04-06-2013, 11:00 AM
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Join Date: Apr 2013
Posts: 4
Erratic Chronometer Readings

Hey fellas please help me understand how I could possibly be getting such wide variations in bullet speed in these 17 shots:

3,152 fps
3,159
3,122
3,167
3,174
3,108
3,093
3,129
3,152
3,115
3,072
3,079
3,144
3,122
3,144
3,152
3,108

These are chronometer readings starting from last shot to first.
High: 3,174 fps, Low 3,072 fps

The results were 5.5 inch groups at 200 yds. I've had .75 MOA at 100 yds with this load, so I had felt pretty confident in it.



Caliber: .300 RUM
Bullet: Berger 185 gr VLD
Powder: 93.5 gr H-1000
Seating Depth:-.175 off lands (Magazine Fit)
Primer: Federal 215
Chrono: Competition Electronics Pro Chrono

Every load was precisely measured, then measured again, cases universally trimmed, etc. I do not go as far as to separate bullets by weight.

Could the jump to the lands cause bullet distortion, thus changes in pressure, resulting in bullet speed variations? I know Berger recommends backing off the lands incrementally until the best depth is found, but this is a hunting rifle and I feel a whole lot better with 2nd and 3rd shot cartridges that fit my magazine.

More importantly, could this possibly be a sign of barrel / chamber issues?

Also, on another note, I have been told that a 185 grain bullet may be a bit light for a .300 RUM... that a 200-220 grain bullet will be more stable and have better ballistics at long distances. Any thoughts there?

Thanks for your help!
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  #2  
Old 04-06-2013, 11:50 AM
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Join Date: Nov 2003
Location: Washington State
Posts: 2,454
Re: Erratic Chronometer Readings

Go with the 215 Berger and H1000 or Retumbo.

Did you have changing lighting conditions during the string? Typically the best conditions for accuracy and consistency of the chrono are consistently overcast (cloudy) conditions. Sunny conditions can result in higher readings. Changing lighting conditions can cause different reading coming from the chrono.

Using a micrometer seating die and a Davidson's seating depth checker to get the most consistent seating depths is something else to consider as well.

Is your bullet seating die seating plug for VLD style bullets? The alternative style has a much larger opening often causing VLD style bullets to make contact with the plug at their tip and not far down on the ogive, which is much preferable for consistent seating depths.

Is your chrono stable and not moving in the wind? Are you sure you are shooting through it at the same exact angle, etc. for every shot (shooting off a bench, or prone off bipod to increase consistency in this regard)? I put my chrono 10 yds out to keep any chance of muzzle blast from moving the chrono and causing a bad reading as the bullet is passing through.
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  #3  
Old 04-06-2013, 12:09 PM
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Join Date: Apr 2013
Posts: 4
Re: Erratic Chronometer Readings

I have a Redding micrometer seating die, and the setting is 'glued' in place so it won't move, so everything is considtant as far as seating depth goes.

Regarding the chrono, is was sunny and the chrono was placed 12-15 feet from muzzle. No postion change or wind to speak of.

The thing is, in other times at the range, my friend shoots his 7mm STW through it and his readings are very consistent, while mine are not. I'm wondering if it could be a problem with the gun. Your thoughts?
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  #4  
Old 04-06-2013, 01:09 PM
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Location: Washington State
Posts: 2,454
Re: Erratic Chronometer Readings

Quote:
Originally Posted by Bvcg1 View Post
I have a Redding micrometer seating die, and the setting is 'glued' in place so it won't move, so everything is considtant as far as seating depth goes.

Regarding the chrono, is was sunny and the chrono was placed 12-15 feet from muzzle. No postion change or wind to speak of.

The thing is, in other times at the range, my friend shoots his 7mm STW through it and his readings are very consistent, while mine are not. I'm wondering if it could be a problem with the gun. Your thoughts?
Weighing every charge carefully, of course? Try changing primers.

The micrometer seating plug (these are also designated as 'VLD' or 'standard', so make sure you have the VLD style or this can cause erratic seating depths) is designed to be used in conjunction with a seating depth checker, Davidsons being a good one. I seat the 300g Berger with a Forstner micrometer die, but I still get some inconsistencies that have to be measured with the Davidsons seating depth checker and then the micrometer seating plug turned a bit to adjust so they are all consistent based off the DAvidsons seating depth checker measurement. U seat them all 5 or 6 thou long, measure, then adjust each one down from there. The plug shouldn't be 'glued'--that's no better than a non-micrometer plug at that point and that's not what a micrometer plug is for.

I would suggest Shawn Carlock's: Reloading for Long Range Hunting

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"And can the liberties of a nation be thought secure when we have removed their only firm basis, a conviction in the minds of the people that these liberties are the gift of God?" Thomas Jefferson - Notes on the State of Virginia

www.wildsidesystems.com - Shelter for Your WildSide - http://www.youtube.com/watch?v=tYwgo...&feature=g-upl
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  #5  
Old 04-07-2013, 02:07 PM
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Posts: 929
Re: Erratic Chronometer Readings

first, replace battery, then try moving chrono to 30' from bbl.
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  #6  
Old 04-07-2013, 05:14 PM
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Posts: 662
Re: Erratic Chronometer Readings

With a 2+MOA group perhaps your chronograph is not all that erratic. In either case you should consider redeveloping a load. While at it, why not try some of the heavier bullets. The 300 RUM is a perfect candidate for the heaviest commercial bullets out there.
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  #7  
Old 04-07-2013, 06:00 PM
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Posts: 2,233
Re: Erratic Chronometer Readings

I have used that chronograph. Sunny conditions with the polished Bergers will give the variations that your seeing. As was stated in a prior post, wait for an an overcast day and set the chrono at 15' from the muzzle. Use a level to make sure the body of the chronograph is level with the barrel, and the bullet passes about 4-5" above the sensors.
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