JBM ballistics drop table ???

[Straight Shooter]

I did a drop table for my 6.5 creedmoor and then went out and shot it at 300 yards. According to my velocity, correct BC, and other pertinent info it says my drop at 300 yards should be 12.9 inches. It is actually 10.5 inches. Does my 20 MOA rail factor in or what else could be the problem? Thanks in advance.

 

[dfanonymous]

No it's off your zero. You rail just gives you more travel in the erector of the scope.

chances are you muzzle velocity is off if all else is correct. BC isn't even worth looking at as it doesn't come into play until around transonic.

You need to change the muzzle velocity until it matches your 2 and 300y drop.

 

[Straight Shooter]

Thanks df. However, the velocity is verified on my chrono. If I match the drop to the velocity I would be shooting the fastest 6.5 creed in the world.

 

[elkaholic]

No it's off your zero. You rail just gives you more travel in the erector of the scope.

chances are you muzzle velocity is off if all else is correct. BC isn't even worth looking at as it doesn't come into play until around transonic.

You need to change the muzzle velocity until it matches your 2 and 300y drop.

Or it coul

I did a drop table for my 6.5 creedmoor and then went out and shot it at 300 yards. According to my velocity, correct BC, and other pertinent info it says my drop at 300 yards should be 12.9 inches. It is actually 10.5 inches. Does my 20 MOA rail factor in or what else could be the problem? Thanks in advance.

Scope height and calibration could account for some of it, but that's a fair amount of variation at that range.

 

[dfanonymous]

The bullet doesn't lie. If that's the case then your zero is off. It has nothing to do with the rail. When it comes to this stuff it does matter that much about the chrono. What's the MV? What's the drop in moa from 2, 3 and 4?

^ or as elkaholic put, it could be another input.

 

[Straight Shooter]

Zero is right on. MV is 2725 fps with a 140 hornaday BTHP with a BC of .580. Really has me scratching my head.

 

[elkaholic]

Zero is right on. MV is 2725 fps with a 140 hornaday BTHP with a BC of .580. Really has me scratching my head.

when it doesn't make sense, I usually find an input error somewhere?

 

[dfanonymous]

Either your bullet is screaming, or you have an erroneous input. Or you scope has some ridiculous error. If you can screen shot your trajectory card, and post it to the thread we could probably see what we see.

 

[Straight Shooter]

I don't know how to post pics.

 

[elkaholic]

I don't know how to post pics.

I don't know how to post pics.

If you have the pic in your phone or computer, just click on "attach files" when you reply and it should bring up your stored pics. Then just select the one you want. The prompts are pretty easy to follow.

 

[Straight Shooter]

Thanks alcoholic, I never knew how to do that.

 

[Straight Shooter]

I think I figured it out. When it cools off a little here I will reshoot. Thanks very much for your help.

 

[aushunter1]

Zero is right on. MV is 2725 fps with a 140 hornaday BTHP with a BC of .580. Really has me scratching my head.

Its beacuse there is theoretical data & real life data.

I dont trust ballistic calculators as there are more/other factors involved they dont know about & I make a drop chart for actuall number by actually shooting those distances at as close to zero altitude.
Then allowing for the variance of altitue/barometric pressure depending on where I am.

 

[Mikecr]

BC isn't even worth looking at as it doesn't come into play until around transonic.

That's funny right there!
Where did you get that notion?

BC is tied to drag, which changes every single inch of bullet travel (not just one region near Mach1).
Now BC itself can be stable throughout, but ONLY if your drag table is custom correct for the bullet.
Maybe 1 in 250,000 shooters have actually tested, created, and built special software to use a custom drag curve.

OP, have you accounted for higher temperatures than the bullet's BC is based on. Most bullet BCs are based on an atmospheric standard like ICAO, Std Metro, or Std. Army. All three at 59degF