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Bad Chronograph

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Unread 03-12-2008, 11:41 PM
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Join Date: Dec 2006
Posts: 422
Originally Posted by royinidaho View Post
My RUM shoots the 300 SMK fairly close to the BC at least to about 900 yards. However, I haven't tested it enough to for the BC to be validated past the first decimal, as in 0.X00

With 101 gr of US869 she goes in the 2730s over my shooting chrony which has been trustworthy except for skipped shots.
What powder are you shooting the 300 with at 101 grains? 92 grains of H100 is all my gun can shoot with out showing pressure signs.
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Unread 03-13-2008, 12:55 AM
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Join Date: Jun 2004
Location: Texas
Posts: 1,056
Bigcat, Roy is shooting US 869. I have a 115 gr load with that powder and the 300 gr Smkhp that chronographs 2780 avg velocity. The load I'm zeroed with is 110 gr of H-5010 and it is the one that averages 2760 fps. It also has single digit standard deviation. With your brake, the effective length of your barrel is probably only 25 1/2" or so. It is not likely that you can get 2770. I think you would have to push it to get 2650. I have 27 1/4" of effective barrel plus a home made brake for an overall barrel length of just under 29".

I routinely set my first screen at 7 yds to minimize muzzle blast and shockwave error. It makes the shooting window smaller, but doesn't rock the skyscreens as violently.

You're not using a B/C of .8 are you? Sierra downgraded the B/C awhile back to .7 something.

If you shoot it across another chronograph, let us know the results.

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A big fast bullet will beat a little fast bullet every time
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Unread 03-13-2008, 05:54 PM
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Join Date: Dec 2006
Posts: 422
Well I cant find anyone locally that has a chrono I can use. I went and shot through my chrono today and in a five shot group it read as so;


So I don't know. I guess all I can do is print out a chart using a lower velocity and hit the range and see if its on. I know where my bullets are hitting at least and the velocity would be around 2650 or so. What a pain. I am going bear hunting in a couple of months and need to get this dialed in for practice.
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Unread 03-14-2008, 10:25 PM
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Join Date: Jan 2004
Location: Blackfoot, Idaho
Posts: 8,505

You seem to have a very reasonable load.

Average Velocity 2749
Extreme Spread 20 FPS or +/- 10 FPS from average velocity
Standard Deviation 8

I'd keep doing what your doing and change BC and/or velocity to make things match up.

Another thing you could try is sending some 22 long rifles through the chrony.

I seem to remember, not sure by a long shot, but something like 1050 or so from some of the 22 rimfires. 1048 keeps coming to mind, but there are questions about my mind, especially the memory part.;)

Maybe someone will chime in with a decent number.
I may be the slowest guy on the mountain . . . . but . . . . I'm on the mountain!
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