Range Report Range card help - Please

[idaho1]

I am trying to build an accurate range card for my new AX .260 and am having some difficulty and need some help.
I loaded my data into the Gseven ballistic program on the internet.... that produced one set of numbers. I went to the field and they did not match up. I re-entered field data and it improved the prediction but it still is off by .1-.2 mils in the mid range (over correction) and .2-.3 under in the 800-850 range.
I repeated the process with the Ballistic FTE program on my iphone. Those numbers were more consistent with field data but still not dead on.
If some of you guys would care to comment/compute with other programs out their like Coldbore or others, that would be graet!!!
The load:
.260 Rem
142 SMK
H4350 41 gr
col 2.80
MV via chrony beta master 2613
2.75"scope height
100 yd zero

temp = 40 F
altitude 3000
BP 29.9
wind 4-6 mph 90"

Field data:
460 yards -2.6 mil
550 yards -3.4
650 yards -4.75 mil
850 yards -7.6

Any help would be greatly appreciated!!

 

[waste_knot]

Heres what JBM has. What's the distance to the crono

 

[waste_knot]

If you increase the BP to about 31 inHG or decrease altitude to 2600 you start to get close.
Are you sure you have 2.75" above the bore centerline.

 

[jbailey]

If I use a 2.0" height over bore (typical for most receivers/rings/scope setups), the field trajectory you saw is nearly nuts on with what JBM spits out, +/- 0.1mil. I'd remeasure your height over bore.

Beyond that, your chrono numbers are probably your biggest source of error, but it looks close enough to make you dangerous if that height over bore number is the source of the error.

 

[idaho1]

Thanks for taking the time!

The scope height is 2.70" with a caliper.
I checked the local weather and the BP was 30.13 when I was shooting so that was a source of error.
The biggest factor I think is the altitude for the 850 truing shots. They were shot downhill. I took this into consideration as far as range but elevation was not modified. The shots were fired at 3000' and impacted at 2600'.

I double checked my chronograph with some .22's that have the fps on the box. It was comparable with two brands.

Thanks again for the help

 

[turbo54]

The FIRST thing you need to do is a full-range scope elevation tracking test, by setting up a tall target at 50 or 100 yards, and ensuring the scope tracks. I don't care if you have a Simmons, Bushnell or Bender.

All exterior ballistic measurments and calculations rely on this being correct, and it shall not be skipped.

It's kind of like tyring a new camshaft in an engine down on power and smoking - before doing a compression/leakdown test. The compression/leakdown tells you instantly whether or not you even have an engine to begin with.

 

[idaho1]

Good advise! I skipped this step.

The FIRST thing you need to do is a full-range scope elevation tracking test, by setting up a tall target at 50 or 100 yards, and ensuring the scope tracks. I don't care if you have a Simmons, Bushnell or Bender.

All exterior ballistic measurments and calculations rely on this being correct, and it shall not be skipped.

It's kind of like tyring a new camshaft in an engine down on power and smoking - before doing a compression/leakdown test. The compression/leakdown tells you instantly whether or not you even have an engine to begin with.

 

[Suasponte]

Here is a copy of the target I drew up for scope confirmation of tracking.
Set the target at 100yds, make sure the target is plumb (I use a string and
plumb bob). Aim at bottom of target and work your way up on your come ups.
Then run it all the way back down and ensure it zero's.
Target has to be set at 100yds exact.

Terry

 

[savages]

Awesome target!
I don't suppose you would be willing to share that file so I can print one?

View attachment 2892View attachment 2893View attachment 2894View attachment 2895

Here is a copy of the target I drew up for scope confirmation of tracking.
Set the target at 100yds, make sure the target is plumb (I use a string and
plumb bob). Aim at bottom of target and work your way up on your come ups.
Then run it all the way back down and ensure it zero's.
Target has to be set at 100yds exact.

Terry