Range Report 338 jbm chart problems

[maccrazy2]

I need a hand with my drop chart. I am shooting a 338 lapua savage 110 fcp hs. lapua fire formed brass, 300gr SMK, 93.6gr h1000, FGMM mag primers. I chrono these at 2780 fps at 70deg and a DA of 7000ft. the sight height is 2.075
I confirmed a good 100yd zero. I shot at a 550yd target multiple times with 7.5moa to get on target. then, 1000yd took 22.25moa the temp was 70deg and da of 7000ft measured on a kestrel 4500.
In JBM I entered the following
.338, 300smk bullet
.381 G7 BC from litz's book
1.7 bullet oal
2780fps
temp 70deg
chrono distance 10ft
sight height 2.075
zero range 100yd
zero temp 70deg
this gives me a 550yd drop of -9.7moa, and a 1000 yd drop of -24.0moa
real world is -7.5 at 550, and -22.25moa at 1000
I have played with the velocity and bc a little but cannot get the 550 and 1000 to come close to real world. Anyone see where I am screwing up?

I am using a beta master chrony for velocity and it is accurate enough to be very close. It has been checked against several other higher end units in the past and I was also shooting my .260 at the same time and it was showing the correct velocity of a proven load that I have been running all year. it is 2800 fps so it is right in the same range.
I was chooting at CRC in colorado and measured the DA and temp and only noted a 6deg rise in temp during the whole session. DA hovered right at 7000ft.
the scope is a nightforce with no known issues and has repeatable tracking. I shot several groups at both ranges as I was there to get good dope so I could work up a good chart for this upcoming weekend. I will be shooting in raton new mexico at the whittington center. The targets will be 700yd to 1800yd ish with the bonus target at over 2k. I do not have anywhere to test real world past 1000yd so I need to rely on a drop chart. Any help is appreciated. Chris

 

[maccrazy2]

Re: 338 jbm chart problems

Afternoon bump

 

[CoryT]

Re: 338 jbm chart problems

You don't indicate here how you input the ATM values into JBM. If you entered DA into the altitude box, you need to check the Std Atm at altitude box. If you entered the temp with the altitude, or unchecked the Pressure is Corrected box, or also entered the pressure, you are not getting correct numbers.

Next item, has the scope been calibrated? Do you KNOW that the value you dialed up is actually correct? Is there an angle to the target that was not entered? Is the Kestrel correctly setup to read DA (referance altitude set correctly).

 

[maccrazy2]

Re: 338 jbm chart problems

Thanks Cory. The scope is gtg. I ran a large box test and verified it is tracking true. There is no angle to the targets. They were shot at a gun club and the distance to the targets were verified with swaro rangefinder.
As for the jbm inputs. I tried the trajectory card in the general trajectory calculators and the trajectory card in the range cards menu.
i tried the srd atmosphere at altitude, pressure corrected both checked and unchecked. Temp set to 70 in all boxes asking for temp and all altitudes set to 7000. I am trying to match real world before making other da cards.
The kestrel was checked against others as it is new so i made sure to verify it that day.
The closest i get is 2.2moa off at 550 and, 1.6moa at 1000
If anyone would run the specs in the first post thru their software i would greatly appreciate it. I need a card out to 2200 yds after i get the card to match real world 550 and 1000 yd drop.

 

[MitchAlsup]

Re: 338 jbm chart problems

After playing around with JBM for a while, and getting it to give 7.5 at 500 yards, changes in various stuff always seem to put the 1000 yards MoA up at 32-34.

I used the 338 SMK [Litz] bullet data. 7000 feet.

The sight heights does not seem to have any big effect.

The pressure does have an effect (a big one) but I could still not get JBM to match your 500 and 1000 yard data points. When the pressure is adjusted to 22.25 MoA at 1000, the 500 yard data reads 5.8 MoA.

My guess is that your 22.25 datapoint is incorrect--maybe by a turn of the turret, or maybe just by the 7.5 datapoint at 500 yards.

The JBM data indicates that this bullet just does not fly that well at 2780 fps, or that if it does, then the 500 yard datapoint is incorrect.

 

[maccrazy2]

Re: 338 jbm chart problems

Thanks for checking. I will have to leave for new mexico early and hopefully can get on the 1000yd range before the shoot and double check the come ups again. Jbm has worked well for my other rifles, it is usally quite close. I do not know what to make of this.