105 gr hunting VLD trajectory Anomaly, HELP.

[tbrice23]

I took my 6mmAI hunting rig to a 600 yard match, I ran the numbers for drops at 600 with a 200 yard zero. JBM said the drop was to be 8.25 moa. My actual zero at the match was 7.5 moa.
.75 moa discrepancy is more than I'd like to have.
Scope is vx6 4-24
1:8 rh twist
105 gr Berger Hunting vld
3275 fps chrono at 11 feet from muzzle
Scope height 1.75" above Center of bore
420' ASL
80° F
.272 G7 BC ,listed in JBM library as Mr Litz' entry (which also agrees with Berger G7 bc.)
I adjusted the velocity to match my dialed drops and ended up with 3345 fps to get 7.5 moa!!!!
I'v chronographed every shot from this bbl. And shot today 3275 fps average for 29 shots 10 SD (shooting chrony, new battery). Dialed back to zero from the 600 yard zero and was dead nuts Zero at 200.

Either my scope clicks are off, JBM is wrong, the 105 hvld bc is very understated, or my chrono is very slow.


Can anyone can run this an a different program and let me know what's up????

 

[Mikecr]

Do you know your scope click value(actual) & did you set this value in JBMs?
This isn't a diff from IPHY to MOA is it?

Just kickin around ideas here;
If JBM was providing a solution in IPHY, but you're actually dialing in MOA, you would end up dialing less than predicted.
Now, most scopes do not actually dial in MOA, but instead generalizations of it(like 'hunter's MOA'). Often they dial a bit more than true MOA. You would have to test your scope & account for this.

Also, it's always a bad idea to lie to ballistic software. You measured your MV, enter that.

 

[tbrice23]

Yes, the turrets and JBM are in MOA.
I know what you mean by lying to the program, I was trying to find out the velocity to match my dialed drops (which wasn't a rational solution ).
I shot a ladder test 2 weeks ago at 200 yards. I should have shot more shots in conformation though.


Another option is, Im using Reloader 23 (supposedly intolerant to temperature ). These are 10 minute matches and rounds were loaded onto a hot chamber, this barrel is a light varmint taper. In all honesty it did get smoking hot. I wonder if there is a possibility of raising pressure that much to gain 65 fps, although I never let the cartridge linger more than 15 to 20 seconds before firing.

 

[westcliffe01]

My 243 AI seems to have gained at least 0.5moa with a proven load from fall/winter to last weeks broiling conditions. I am using Hodgdon H100V, not reputed to be an "extreme" (stable) powder. I fire very few shots so barrel/chamber heat is not a factor.

 

[tbrice23]

I bet you are getting good velocity with 100v.??

It isn't really feasible to chrono at a sanctioned match. But I'll bet the chamber temperature might be the variable that fits the equation.

 

[Greyfox]

Not sure if this is your issue but I have always considered the chronograph the weak link. I had this same issue with a 70FPS(low) discrepancy with actual impacts with my 6.5x284. I was using a ProChrono. ES was very tight and results were always the same, When I retested with my Ohler 35 and Magnetospeed, everything tied together with velocities matching my actual drops.