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Chronograph distance and true MV

 
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  #1  
Old 09-23-2008, 01:30 AM
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Chronograph distance and true MV

How can one compute the true muzzle velocity (or at least closer to it) when using a chronograph that's not actually at the muzzle? I remember reading about this in the past, but cannot find the article. I routinely set my chrony at 11 feet from the muzzle (closer, and the muzzle blast from my 300 RUM usually knocks off the diffusers).

My MV's are not up to what's stated in load manuals, and way off what Quickload computes them to be. I understand there's many variables besides the powder type and charge, primer, barrel length, and projectile. The reason I'm wondering about the true MV vs. what I get at 11 feet is because I'm seeing some signs of high pressure. Some cases have ejector marks on the head, but I've never noticed a more difficult bolt lift or any flattening of primers.

Two loads in question are: .300 RUM = 92.5 gr RL-25, Fed 215, 180 gr. Nosler Accubond (3100 fps) and 180 gr Swift Scirocco (3100 fps); and .300 SAUM = 61.2 gr. IMR 4350, Fed 215, 180 gr. Nosler Accubond (2800 fps). I fellow reloader ran Quickload data on the SAUM load and informed me it was way over pressure. Haven't checked the RUM loads with Quickload.

Just trying to read and gather as much info as I can before I go and start tweeking what I thought was a satisfactory load at modest MV for the cartridges.

Thanks
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  #2  
Old 09-23-2008, 01:40 AM
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Re: Chronograph distance and true MV

JBM - Calculations - Trajectory

Enter your data there...at the bottom of the page it will give your muzzle velocity...or pretty close to it.

Its usually about 6 fps over the chrono reading on most rifles.
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  #3  
Old 09-23-2008, 04:10 AM
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Re: Chronograph distance and true MV

300 Rem,

When in the field, I use my PDA which runs LoadBase 2.0 and right there I get the corrected muzzle Velocity.

This two screen shots from my PDA show how the program does it:



On the left it shows the info. required and on the right the corrected Velocity.

The 1 is 1foot internal distance between the screens.
The 12 are 12 ft from muzzle to the first screen (other applications do it to the center of the screens)
The rest of the info is obvious and you can see the correction was 17.2 ft/sec. If I use a BC of 0.631 the correction would have been 6.4 ft/sec as previously mentioned.
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  #4  
Old 09-23-2008, 08:53 AM
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Re: Chronograph distance and true MV

That's a pretty cool feature of LoadBase 2.0, Eaglet. I wonder how it compares to the method I've used, which I'm sure is not so accurate, but if you're off a couple feet persecond, from what I've seen, it makes essentially no difference. I've messed around with MV in Sierra program or Exbal (little more limited with Exbal in 'reporting distances') and when the speed at the interval of say 20' (or 30' in the case of Exbal as 10 yards is as close to the muzzle as it reports, I think), then just look at the 0 distance line to get your MV. Basically, match your chrono speed at 20' for instance to the 20 ft velocity on the drop chart and then check what the velocity is at 0 ft according to that drop chart. Hope this helps.
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  #5  
Old 09-23-2008, 06:10 PM
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Re: Chronograph distance and true MV

jmden,

I see what you're saying and does make sense. Theoretically it would work, the pain comes in when you start playing with it; ex: 4 yards = 12 ft, set the chronograph at 12ft, if your reading is 3000 ft/sec your chart table would have to read at a distance of 4 yards 3000 ft/sec; then you look at the zero line and what ever it says that should be without doubt your corrected muzzle velocity.
I tried to go to the motions using different programs and found it kind of painful. Most of us including you would probably hate to be frustrated after a while because you eventually would have to keep changing muzzle velocities at 0 yards until at 4 yards it matches the chronograph velocity. Not to mention that most programs are not willing to give you that info at 4 yards.
It does work though!!!
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-----------------------------
HEBREWS 13:8
Jesus Christ the same yesterday, and to day, and for ever.

Our Lord Jesus said that as it was in the days of Noah and
also as it was in the days of Lot so it shall be in the days...
It's happening again!!! God sent to us His prophet, and His Word
to this generation and we once more are rejecting it as was prophesied!!!

---> As promised, God Sent His Prophet to us!
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  #6  
Old 09-24-2008, 01:19 AM
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Posts: 13
Re: Chronograph distance and true MV

Thanks for the advice. Eaglet, obviously it makes little difference having a chronograph at 12 feet according to LoadBase 2.0. I appreciate the input.
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  #7  
Old 09-24-2008, 02:01 AM
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Re: Chronograph distance and true MV

Quote:
Originally Posted by 300 Rem View Post
Thanks for the advice. Eaglet, obviously it makes little difference having a chronograph at 12 feet according to LoadBase 2.0. I appreciate the input.
The difference changes are based greatly on BC, the higher the BC the lesser the change and the lower the BC the higher the change.

The ballistic application called Ballistic Explorer will correct to Muzzle Velocity using all the conditions like temperature, altitude, barometric pressure, humidity but in that short of a distance what the BC changes does not make much of a difference. I always do use corrected Muzzle Velocity!
__________________
-----------------------------

-----------------------------
HEBREWS 13:8
Jesus Christ the same yesterday, and to day, and for ever.

Our Lord Jesus said that as it was in the days of Noah and
also as it was in the days of Lot so it shall be in the days...
It's happening again!!! God sent to us His prophet, and His Word
to this generation and we once more are rejecting it as was prophesied!!!

---> As promised, God Sent His Prophet to us!
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