I seem to recall Darryl or one of you other LR pros calculating a different BC on 220 MK's than what Sierra publishes. If one is consistently travelng 3175 any thoughts on this would be appreciated. Thank you
The big unknown is the effect your barrel has on the bullet. I believe that Sierra bases their BC on actual shooting over chronographs. That is why their BC's vary with velocity. Most other companies base theirs on a shape formula and can vary alot in the real world.
The only way to get a true BC for a bullet is to fire it from your rifle and measure its speed or drop over known distances.
Ian M. has been doing some of that and might have some results to compare with yours. I have only found the published BC to agree once out of a lot of different bullets and rifles.
I'll be running some through the Oehler 43 out of a 300 Ultra in the next few weeks, it will give a true BC for that speed and conditions.
If you have a GOOD cronograph and if you start with perfect 100 yard zero groups, then go to between 500 and 700 yards and measure the ACTUAL drop. Compare to a drop chart at the temp etc during shooting and now you have something to go with to click on out beyond this too.
I have always used a sheet of plywood standing vertical covered with butcher paper. Remember to measure the center of your groups for accurate results.