Shot-to-Shot Variation in Muzzle Velocity (MV) and Ballistic Coefficient (BC)

TRG65

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So if I read this right, having a load with a velocity SD of 5 and shooting pointed bullets which is about .5% (and you say equates to the same variance as an SD of 5) would yield in real world results the same amount of variance as your initial SD of 10 chart shows? and likewise SD of 10 with normal bullets or 1% BC variance would yield the same real world variability as your initial SD of 20 on the chart?
 

DocUSMCRetired

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So if I read this right, having a load with a velocity SD of 5 and shooting pointed bullets which is about .5% (and you say equates to the same variance as an SD of 5) would yield in real world results the same amount of variance as your initial SD of 10 chart shows? and likewise SD of 10 with normal bullets or 1% BC variance would yield the same real world variability as your initial SD of 20 on the chart?
Depends on the bullet manufacturer, however those who have received PDMs actually get a print out with this data. I have seen some manufacturers or "match" bullets as high as about 3% BC SD, and others as low as 0.3%.
 

Quintus

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The more I learn the more amazing it is we can hit anything past 600 yards. When you factor wind and component variation (bullets, powder, primers, and cases) barrel fouling, temp and humidity changes and on and on. Amazing how far things have progressed in the last 25 years and the average shooters groups have shrunk.
 

DocUSMCRetired

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The more I learn the more amazing it is we can hit anything past 600 yards. When you factor wind and component variation (bullets, powder, primers, and cases) barrel fouling, temp and humidity changes and on and on. Amazing how far things have progressed in the last 25 years and the average shooters groups have shrunk.
It is also important to remember that when we evaluate groupings that we are notating the outermost ring. However 80% of your shots should be less than 66% the size of that ring.
 

Bravo 4

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This would also apply to the effects of wind too, just on a smaller scale.
More reason to run a hotrod (fast) cartridge😁 If you run a bullet to higher velocity will it not mitigate some of the effects. Example: At 1000 yards
If I run a 175 SMK (.5 BC) at 2600 out of a .308, a velocity difference of 20 fps between shots makes about a 7” difference in drop. Run that bullet out of a .300 RUM at 3200 fps and the difference is half that.
Or am I (and my AB program😁) wrong in this?
 
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Quintus

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This would also apply to the effects of wind too, just on a smaller scale.
More reason to run a hotrod (fast) cartridge😁 If you run a bullet to higher velocity will it not mitigate some of the effects. Example:
If I run a 175 SMK (.5 BC) at 2600 out of a .308, a velocity difference of 20 fps between shots makes about a 7” difference in drop. Run that bullet out of a .300 RUM at 3200 fps and the difference is half that.
Or am I (and my AB program😁) wrong in this?
Faster is always better...most of the time.
 

DocUSMCRetired

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Balancing act. Higher BC = Less Wind Drift. Higher Velocity = Less Time Spent In Wind. Lag time is a great subject to research.

Perfect example are the 22-250s where they perform beautifully at certain distances. But you are shooting low BC bullets so that speed falls off quickly and the advantage with it. It is all about balance for your particular application.
 

Barrelnut

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Questions: What causes the BC difference in the bullets?
Is it physical variation like weight and bearing surface length, meplat variance of each bullet?
Or is the BC difference dependent on the muzzle velocity variation?
So sorting bullets and meplat trimming does help!?
 

Bravo 4

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Balancing act. Higher BC = Less Wind Drift. Higher Velocity = Less Time Spent In Wind. Lag time is a great subject to research.

Perfect example are the 22-250s where they perform beautifully at certain distances. But you are shooting low BC bullets so that speed falls off quickly and the advantage with it. It is all about balance for your particular application.
I was just using that as an example to my point. I do shoot the 175 SMK in a .308 but have no choice in the matter, it’s issued. I have 215 and 230 Bergers for my RUM. I think of rifles as a system and no one component will maximize it’s potential, it’s a balancing act.
 

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