Velocity testing ---- contradiction

[carswellb58]

I've been velocity testing a new barrel and have hit a very peculiar contradiction. I'd really be interested in any thoughts on the contradiction I outline below:





Having put a new barrel on my 6.5x284 Norma I took it out this morning to get a velocity check.

My ammunition (hand loaded) is as follows:
Brass - Lapua
Primer - Federal 215m
Powder - H4350 with a charge of 50.0 gr.
Bullet - Berger 140 gr BTHP VLD Hunter (G7 BC - 0.307)
Rifle: Custom build with a Stiller receiver, Jewel trigger, and a Brux barrel (brand new but thoroughly cleaned after chambering)

I fired 10 rounds and measured velocity with a LabRadar doppler chronograph. Here are the velocities I got:

1) 3014
2) 3025
3) 3002
4) 3000
5) 3020
6) 3028
7) 2998
8) 2990
9) 3014
10) 3006

This yielded an average velocity of 3010 with an extreme spread of 37 and a standard deviation of 12.4. I consider these results acceptable although I prefer an ES of 30 or less and an SD of 10 or less. Still, I was all right with these results.

The contradiction arose when I began to shoot steel. I used the Applied Ballistics application (from Brian Litz) to generate my ballistic table in preparation to shoot 4" steel discs at 200 yards, 230 yards, 250 yards, 300 yards, and 330 yards. The table I generated gave the following drops:

200 yards - 1.1 MOA (use 1.0)
230 yards - 1.6 MOA (use 1.5)
250 yards - 1.9 MOA (use 2.0)
300 yards - 2.9 MOA (use 3.0)
330 yards - 3.5 MOA (use 3.5)

To hit a 4" disc I can aim dead center and not be off more than 2 inches. This calls for sub-MOA accuracy at everything longer than 200 yards, something I am confident I can do.

*HOWEVER* I could not get reliable (meaning repeatable 3 times) hits until I added .25 MOA to all distances, hence my dope became

200 yards - 1.25 MOA
230 yards - 1.75 MOA
250 yards - 2.25 MOA
300 yards - 3.25 MOA
330 yards - 3.75 MOA

****Hits with these drops were quite reliable !! ****

Now - here's the problem. I went back to the Applied Ballistics app and by trial and error revised my stated MV until I got a close match at all the tested distances. The best match came at a Muzzle Velocity of 2920, a full 90 fps slower than measured with doppler radar (????). It's hard to imagine the LabRadar is that far off, but I trust the ballistic calculations in Brian's application, so I'm totally confused. I'd very much like to hear opinions as to what's happening.

 

[velvetant]

What did you use to get your atmospheric conditions to plug into the app?
Was your 100 yard zero perfect or off a little?

 

[carswellb58]

I set my 100 yard zero an hour or two before I began my velocity testing, but I suppose I could have been 1/4 inch off on that. I'll recheck. A tight group 1/4 inch low of dead center could be overlooked and that would do it. That's the most probable root cause now that I think about it. Thanks for the comment. A second set of eyes always helps!!

 

[dfanonymous]

it can be anything. and headwind, bc needs to be trued down, turrets tracking needs a correction factor added..could be just how you shoot, how you zero'd etc. seems to be a common question here, and i tell people the same thing...do what you did...get that real dope and true it to the calculator...works better the farther out you go.
also a heads up..if you mess with the bc...definitely go out as far as you can before you mess with it, because it will mess with other distances..