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Old September 11, 2002, 06:42 AM   #1
klh8
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Shooting through a chronograph

Was at the range yesterday and my buddy took his chronograph.My load with the 125gr sierra out of the .308 is 3000fps according to the reloading manual.
The chronograph was clocking the load from 2860 to 2904 fps.
Is this difference typical according to shooting and weather conditions?Chronogarph was set up around 3.5 to4 feet from muzzle of my rifle.
Thinking,maybe his chrono was not accurate.
Just curious.
Thanks fellas
Paul
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Old September 11, 2002, 06:58 AM   #2
NG Bruce
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That is absolutely normal. I just chronographed some 45 Colt loads that the manual listed as 1398 fps avg - I got 1276 fps avg. The temperature, distance from the muzzle (I use 8' for revolvers and 10' for rifles), barrel length of your gun; all of these make a big difference. Some reloading manuals use a pressure barrel and 26" barrels - they measure different than barrels on field guns. Powder lots, primer lots and bullet lots can make a significant difference. Check out an old Hercules data manual (I have one from 1984 and one from 1990) they list the exact same data in several of my cartridges in the 2002 manual - obviously the data has not been re-shot to take into account the changes in the powder. I use the Hornady 165 grain BTSP in .308 in my .300 Win Mag. One lot of the bullets gave me 100 fps more velocity than the ones used before or after - seems the ogive was different and was touching the leads - spiking the pressure. The chronograph helped me pick up on the change. You have to work with a chronograph a while to learn to use the data, but I wouldn't dream of load development anymore without one. NG Bruce
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Old September 11, 2002, 09:36 AM   #3
Clint Tickler
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I second Bruce's comments. I have a .308 in a Rem VLS and a 788-.308. Shooting a 125 gr Ballistic Tip with 48.5 grains of RL-15 out of the 26" VLS gives me an average of 2980 fps where the same load out of the 22" 788 clocks at 2812. I'm 10' from the chrony.
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Old September 11, 2002, 10:24 AM   #4
braindead0
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Luckily, so far I haven't shot through my Chrony...always above..
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Old September 11, 2002, 10:27 AM   #5
Poodleshooter
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Velocities printed in manuals often have very little to do with actual results in your rifle. Ignore them, other than for reference, and keep using the chronograph.
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Old September 11, 2002, 02:50 PM   #6
Mal H
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As they said above, your measured velocities sound normal. Don't take the figures from a manual as anything but a guideline. The manufacturers often use universal receivers and longer than normal barrels hence they will get higher velocities. But even with the same length barrels or even the exact same test gun as you are using, a fairly large difference in vel is not at all uncommon.

However, the main thing I wanted to point out is what the others have implied, you are shooting too close to the chrono. Four feet is way too close because you can get an occasional false reading from the gases starting and/or stopping the clock too soon. Ten to 15 feet is a good minimum distance for rifles. Depending on the caliber, 10 feet is also a good compromise for handguns.
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Old September 11, 2002, 09:05 PM   #7
klh8
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Thanks for the replies. To what Mal H said,the muzzle blast on occasion would disconnect the rods from the front plastic hood(?)Do not know much about the chronograghs.Need to get one.
Regards
Paul
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