Muzzle velocity consistancy

Montana2015

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May 1, 2018
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My son purchased me a Caldwell Chronograph for Christmas and I tried it out on my reloads this week. I used two different recently cleaned rifles /two different calibers. All bullets where the same and all powder charges are measured, using a basic RCB manual scale and a powder tickler. Here are my FPS velocities. 6mm= 3307, 3352, 3270, 3270, 3282. And for the 279 WSM=3274, 3318, 3320, 3333.

Here are my questions; What would cause the variance in velocities? Chronograph problems, Scale problems, Human problems? Is an electronic scale more accurate? And the big question, Is the variance enough to be of a concern? Thanks for the help and feedback.
 

Don Fischer

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One really good shooting load I had had a 100fps variation in velocities. I think it's easy to over think this stuff, look for group's, nothing else matters!
 

Addicting

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I switched to a electric scale and measure out to the nearest thousands on powder. I also switched to bench rest primers.
I weight sorted everything. Most of my groups FPS Extreme spread is under 20 FPS. With a large percentage of those being single digit spreads.

With that being said for what short distance these rounds are for it is completely unnecessary. I just get really bored in the winter.

If your getting good groups and are staying under 400, I would say your fine.
 

Mtnhunter1

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Montana2015,,,, There is nothing wrong with your chrono! The first round fired, from a clean barrel, will always show lower velocity. A clean barrel provides less drag on the exiting bullet. This less drag produces less pressure, so less velocity. After that first shot, the barrel is considered fouled. A fouled barrel will produce consistent velocity until the fouling build up gets to severe. Your 6mm 30fps and 270wsm 15fps deviation would be considered very good for hunting reloads. Many factors will effect velocity deviations but I would consider your results very acceptable.

If you don't mind me asking, what bullets were you shooting with your above mentioned loads?
 

std7mag

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Depends.
Are you looking for minute of deer at 100 yards?
Or 1/2 MOA at 1,000?
For deer/elk/antelope out to 300 yards your ES won't matter too much.

If your trying to shoot at longer distances, 600 yards or better, they are too large in Extreme Spread and Standard Deviation.
 

Montana2015

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Montana2015,,,, There is nothing wrong with your chrono! The first round fired, from a clean barrel, will always show lower velocity. A clean barrel provides less drag on the exiting bullet. This less drag produces less pressure, so less velocity. After that first shot, the barrel is considered fouled. A fouled barrel will produce consistent velocity until the fouling build up gets to severe. Your 6mm 30fps and 270wsm 15fps deviation would be considered very good for hunting reloads. Many factors will effect velocity deviations but I would consider your results very acceptable.

If you don't mind me asking, what bullets were you shooting with your above mentioned loads?
I am using Barnes TTSX 130 grain with the 270 wsm and Spear 85gr boat tail with the 6MM
 

Mtnhunter1

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I am using Barnes TTSX 130 grain with the 270 wsm and Spear 85gr boat tail with the 6MM
The velocities for your bullet choice fall right in where they should. I don't even turn the chono on until I fire a fouling shot or two. After the fouling shots, your velocity should even out to the point of getting an effective evaluation of your reloads.
 

Brian in Montana

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Very interesting stuff. I wouldn't have thought a patina of copper and carbon would effect velocities like that, but I could believe it. My most accurate loads have an ES of between 20 and 40 fps or so, but I'm generally getting that chrono data in a fouled barrel.
 

ImBillT

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As your cleaned barrel fouls your velocity spread will settle down unless the load is worse than the change the fouling causes, so shoot 10-20 rounds after cleaning THEN check your velocities. Ten shots is better than five for checking SD and ES. At 100 yards you can get incredible groups with very high SD and ES. By 200 it starts to show up a bit, but single digit SDs are not really meaningful outside of competition until your shooting farther than most would be comfortable shooting at game. When I started shooting mid-range F-class matches I shot some decent scores at 300 with a load that wouldn’t hold together at 500 and 600. Chronographs certainly have value, but the paper will tell you the story as well, particularly for hunting loads. If you have excessive vertical at 300+, and when windy the string is angled from high into the wind to low away from the wind, then your load has excessive velocity spread, but is otherwise well tuned(good seating depth, neck tension, average velocity etc.) and you need to do the various things that reduce velocity spread. If your groups appear more random, then you have other issues to contend with. If your gun isn’t shooting almost all groups under 1MOA and 100 and 200, then I wouldn’t worry too much about velocity spread.
 

Dan O

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One thing to check is your brass being from the same lot and brand. Using different brand can cause up to a 100 fps change in velocity with a given load. I experienced this with my 270. I agree with the clean bore shot being different from the fowled bore shot.
Dan
 

FLS

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The problem may be your chronograph.
All I use for rifle is a magneto speed or a labradar. The others just have too much variation due to lighting, setup etc.
 

std7mag

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I just noticed its 6fps
I'm jealous, of the Dakota...
My 7mm-08AI shooting 140gr Berger VLD Hunting, with PPU cases, Fed 210M, Alliant PP2000 would net me 7fps ES, 2fps SD over 10 shots.

The lower you can get both those numbers, the better off you are.
 
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