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Bad Chronograph

 
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  #1  
Old 03-11-2008, 09:54 PM
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Bad Chronograph

I shoot a 338 ultramag with 300g Sierra Matchkings. My chrono says 2770 f.p.s. consistently. I follow my drop chart and it hits low about 5" at 500 yards, 6" low at 600, 7" low at 700 etc. It seems to shoot as if the velocity were more at 2650 f.p.s. or so. Could the chronograph be faulty? I am using a "shooting Chrony" brand chronograph.
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  #2  
Old 03-11-2008, 10:24 PM
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Best way to find out is to find someone elses chrono and put a few rounds thru it. Preferably with yours right behind his and shoot thru both at the same time.
Dave
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  #3  
Old 03-11-2008, 10:28 PM
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have you thought that the chrony might be right and that the BC on your bullet isnt what you thought it would be. i know that i use berger 168 and they say that they are .648 and i have done alot of testing on this gun bullet combo and its more like .64. this might be your problem if its not the chroney
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Old 03-11-2008, 11:15 PM
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Big cat, what is your barrel length? I shoot 2760 fps (with the start screen at 21 ft) with 300 gr smkhp's in my .338 x .378 with a 27 1/4" barrel, so that sounds in the ballpark. If you can shoot through an Oehler 35/35P, try that, but if the actual drop is greater than it should be, then the downrange velocity is for sure lower than you're using in the drop table. Could be muzzle velocity (chronograph reading too high) or lower B/C or both.

If you have a rifle that shoots on the drop table, try it through the chrony and see if it reads higher average velocity than the drop table velocity.

Good luck, Tom
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  #5  
Old 03-11-2008, 11:40 PM
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Quote:
Originally Posted by specweldtom View Post
Big cat, what is your barrel length? I shoot 2760 fps (with the start screen at 21 ft) with 300 gr smkhp's in my .338 x .378 with a 27 1/4" barrel, so that sounds in the ballpark. If you can shoot through an Oehler 35/35P, try that, but if the actual drop is greater than it should be, then the downrange velocity is for sure lower than you're using in the drop table. Could be muzzle velocity (chronograph reading too high) or lower B/C or both.

If you have a rifle that shoots on the drop table, try it through the chrony and see if it reads higher average velocity than the drop table velocity.

Good luck, Tom
My barrel is 27" with the muzzle brake. I set my chrono at 10 feet from the muzzle. I am just using Sierras BC info so I don't know.
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  #6  
Old 03-12-2008, 07:54 PM
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i'm not gonna say your rum can't shoot 2770 with a 26" barrel, but i'd bet it's pushing brass real hard to do it. 2650 would seem like a much more realistic speed to me.
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  #7  
Old 03-12-2008, 09:15 PM
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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.
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