Powder change. Why the inconsistencies.

[Slofr8]

Hey there,
I usually figure things out here by searching but this is different.
I have some property behind the house that allows me to shoot out to 600 yds. I have steel plates every 100 yds. Distances were google earth checked and paced off. Should be very close.
I started reloading for my .308 last winter using 168 smk's on Varget. I reloaded in .5 grn. increments and found the/an accuracy node at 42.5 gns. I have Shooters ballistic calculator but I do not have a chrono.
I then started shooting at steel and played with the MV until things matched up great. I set things for 60 deg. F and needed no adjustments at this distance from 40F- 80F. Elevation was near perfect and matched the calculator. Figured I was stylin'.
Then, I couldn't find Varget and tried IMR 4064. Accuracy node again at 42.5 (coincidence) and a little high at 300. Satisfied, I bought more 4064 and shot up most of my Varget.
Yesterday I pulled out the 4064 rounds and started shooting. Conditions were 40 F and rain. I found elevation corrections for 300-400 but could not get them to match Shooters solution for 600. .4 mills low. I easily adjusted for it but then things were off at closer ranges. I played with MV but no dice. Now I can and will make my own range cards but I'm wondering if there is something I'm missing. I'm not a match shooter and this is the only intended purpose for this rifle. This is the first time I shoot in this amount of rain if that matters.
Thanks, Dan.

 

[m1match]

IMR 4064 is more temperature sensitive than Varget. A general rule of thumb that works for many powders is 1.2 fps of velocity change for every degree of temperature change. For example, with Reloader 15, that rule of thumb works well enough so my Shooter solutions match my real dope. The other thing that could make the difference is you didn't have your environmental date correctly input into Shooter. Besides temperature, you have to input the absolute barometric pressure. A common mistake is people put in the corrected barometric pressure (corrected to a certain reference point, pilots use this number to input into their altimeters so their altimeters will give accurate altitude). For our ballistic software, we need to put in the uncorrected absolute barometric pressures, or the station pressure. In a Kestrel or other personal weather station, you get this by setting the reference altitude for the barometric reading to 0. If you didn't do this, the Shooter could easily have been off my as much as you found.

 

[Slofr8]

Thanks m1match. I switched Shooters from "simple" to "advanced". I'm more a simple kind a guy. I input corrected BP and 100% humidity and it did change things a little. Actually very good out to 500. 600 was still off and conditions are about the same as yesterday. One thing I noticed is where my 600 yd. target is, the wind changes due to the trees. I would go out on four wheeler to see because I couldn't make out the 600 well enough in the rain. While spray painting the steel I had a left to right wind out to 500 but quite a tail wind at 600. Maybe more so now that the trees are fully leafed. I would spray paint and it would go straight away and down. Not saying this is the whole deal but I might have to wait for a calmer day for more testing. I really enjoy the challenge of trying to figure out the wind. It might be this weekend was not the best time to work with a new load.
Thanks. Dan.

 

[jwp6114]

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