Model not matching the real world

[Chanonry]

OK this is melting my head.

Shooting an AX 308, NXS 5.5-22 x56, 168gr TAP.

Bullet is about 0.25 - 0.5" high at 105 so zero'd about 130 yards.

Shooting left over little bits of clays at 311 yards consistently with 3.25 MOA. MOAR reticle. Same dope dialling or holding. Not an issue.

I have chrony'd the ammo previously at 2550 fps. I am highly suspicious of this number so ignored that this time out and tried to get a JBM trajectory to match the real world, using the Litz BC's. Sight is 1.875 " above the barrel cl.

Varying the MV I can get a predicted drop of 3.4 MOA at 3000fps. Clearly not correct. If I run the advertised MV of 2700 fps, then the predicted drop is 4.4 MOA.

I am shooting at 22x magnification.

What's going on ????

 

[Greg Langelius *]

If the model doesn't match observed performance, the model is wrong.

Rather than varying inputs based on ballistic performance, try varying inputs based on the optical sight line. You should be varying inputs like scope height and such, and you will probably find a better correlation between predictions and observations.

To confirm scope height, measure the bolt diameter and divide that in half, add the gap between bolt and scope tube, and add half the scope tube diameter. That should give you a very close estimate for the height of the optical axis, which is the value that's needed for scope height (above the bore axis).

Also, if the Chrono is close enough to the muzzle, it may be measuring the speed of the gas in the muzzle blast, which catches up and passes the bullet before it dissipates to an undetectable degree. The net effect of placing the Chrono too close is a Chrono reading that is faster than actual MV.

The Chrono should be at least 12-15ft downrange from the muzzle, and 20ft is even better. Putting it further out there places a premium on proper alignment between the bullet path and the Chrono's timing gap. Get it wrong at the price of blowing away your chrono.

Greg

 

[Chanonry]

Agreed on the model being wrong, so I am trying to fix my model. There are only so many variable which is why I am stumped !!

I will remeasure scope height.

Chrono was 20' away. Still not convinced that the numbers are right.

 

[Chanonry]

Good shout Greg. Scope height miles out. There is a solution that maps drop at 100 yards and 311 yards but at 2750 fps not 2550 fps.

Chrony for sale….

 

[shovelstrokeed]

LOS angle - line of sight, might be messing with your data as well.
There is a great app for the iPhone called tilt meter that will give you the info real quick like.
I set it on my turret top pointing down range. Do a quick zero of the angle using its calibrate feature. Just face one way and then face the other.
Now back on the rifle and, with my bipod at my normal shooting height, I level the tilt on the scope. Never mind where it is pointed. Use a stack of books, note books (thinner) 1/4" plywood squares or squares cut from floor tiles. Get the damn thing level.
Now get behind the gun and acquire the target with the iPhone sitting on the scope. Have a buddy read the tilt angle and input that into your data.

I shoot factory ammo and pretty much don't have much issue with the Hornady 120g A-max I'm using in my 6.5CM gun. Velocity of 2910 seems spot on.

I'll be out there this weekend. Only have the difference between 100 and 200 for now to play with my app, would prefer 600 or 700 but it just ain't there at the local range. BTW, I use Ballistic AE which uses JBM data for calculations.