Range Report JBM/ .243/ Amax

[H5N1]

I run a 105 amax with 41.5 of H4350 26in bbl. Last time chrono was averaging around 2990-3000ish fps, it was still in the 90's out.
This afternoon I range a doe 432yds,very slight down hill. My cold bore shot seems to be a little high more times than not, so I dial up 6moa thinking this should be close enough. Shot goes over her back, so I drop 3 clicks and its perfect. WTF? to make that work on JBM I'm running 3100 fps, I feel certain that is not the case.
This is a M700 in an AICS with a 20moa NF base and a 16xSWFA/SS with low Burris Extreme Tac. rings.
What am I missing?
I was just at 100yds the other day.
Also, the AMAX worked great nice exit wound a little larger than a half dollar. This is the first body shot I have taken on game with it.

Thanks for your help, and comments.
Kyle

 

[BryanLitz]

Re: JBM/ .243/ Amax

Kyle,

Congrats on your hunt.

I'll take a stab at guessing what your problem might be. Assuming you're giving JBM all the right inputs, what could cause your observed drop to be so different from calculated?

Many times the answer to this question is that the scope adjustments aren't exactly as advertised. For example, if your crosshairs actually move .28 MOA per click instead of .25 MOA per click (very possible), you will have .72 MOA of 'extra' elevation over a 6 MOA correction, requiring 3 clicks down to be zeroed.

I advise the 'tall target test' to check your scope adjustments. Basically, you set a target up at 100 yards and shoot a group. Now make a scope adjustment, say 10 MOA and shoot another group at the same aimpoint. Measure the distance between the two groups and see if it's truly 10 MOA. If not, you have to establish and apply a correction factor to your scope when making adjustments.

I also advise double checking all inputs into JBM. Verify your MV if possible. Be careful to account for atmospherics properly.

Another thing, 3 clicks at 432 yards is only 3.4 inches. If your first shot went over the deer, and a 3.4" adjustment put you in for the second shot, I suspect your cold bore zero is very high as you observed.

Just some ideas to kick around.

Good luck,
-Bryan

 

[H5N1]

Re: JBM/ .243/ Amax

Bryan,
Thanks for taking the time to answer my question, I will try that test this afternoon.
Being that you responded to this I would also like to say I appreciate the work you do to advance the science of shooting.
Kyle

 

[BryanLitz]

Re: JBM/ .243/ Amax

Thanks Kyle,

Let us know how it goes.

By the way, Gus McRae is one of my favorite characters of all time. Some other good quotes:

"... The first man that comes along that can read Latin is welcome to rob as far as I'm concerned; ya see, I'd like the opportunity to shoot at an educated man once in my life"

and

[in response to being asked by the hooker if he cheated for a 'poke']
"Well I won't say I did and I won't say I didn't but I will say this: a man who wouldn't cheat for a poke, don't want one bad enough!"

 

[H5N1]

Re: JBM/ .243/ Amax

Shot this afternoon, cold 150round fouled bore shot at 100yds went 1MOA high. My zero for 100yds was 2 clicks high also after completing the group. I don't shoot at 100 very often the latest reason was being sociable with the family while they zeroed for deer season, until this episode. A check every once in a while may be in order.
Now the "dope" is good and verified to 600 now.
I know cold bore shots are controversial but for what ever reason it is aggravating and hard for me to trust it when the time comes, but I swear that first one just goes a little high.
Time will tell I will be shooting my first from now on at 100 just to see.