I did some shooting at 420 yds and 545 yds using my target knobs for the first time. They are set up for MOA clicking. My scope has 1/4 min. clicks and the knob scales match up perfectly as I click my way around. I used 1/4 min. clicks calculated from my Sierra ballistics program given the altitude, baro pressure, bc, known MV, etc. I'll have to do some more shooting, but my POI seemed to be the equivalent of about 3 clicks below target center on average. When I played with the ballistics program and lowered the MV from 3285 to 3100 (180 Accubond on what was a stock 300 RUM that I've accurized and carefully loaded for), then it appeared that the clicks would've put me right on target.
The one 420 yd 3-shot group was 1 5/8" and about 2 3/4" low--right on left to right--this with 10 clicks up given my zero. The 545yd groups spread out some (4 5/16" but spread left to right with me trying to guess the crosswind) due to me getting to the range too late in the morning and playing with winds that seemed to pick up beyond the 420 yd mark for some reason. The 545 yard groups were about 4" low.
One thing I can think of is that I estimated barometric pressure off of this chart ( http://www.sablesys.com/baro-altitude.html
) and didn't take a direct reading. But even drastically changing BC or BP on the program didn't net results that lined up with what I saw on paper. Changing MV down 185fps seemed to make things line up with what I saw on paper. Hmmm...
I called Sierra and the tech just said that there's nothing wrong with the program, which I hadn't intended to (and didn't) communicate. I asked him for help figuring out where my input data might have gone wrong, but he said that they were really too many variables to figure it out. He did mention that my barrel might be such (rough, etc.) that it's changing the BC of the bullet. He also mentioned that Nosler doesn't give various BC's for the various speeds the bullet is going like Sierra does and that this might contribute to the apparent error. (BTW, my notes say that the techs at Nosler say that the .507 BC for the 180g .308 Accubond is @ 3000fps.) When I've talked to Nosler about this, they say that it's so negligible as to not be a problem.
I'm also very new to the "clicking" game although I'm quite familiar with my ballistics program and am pretty sure I can count clicks!
Ideas would be appreciated.