DA vs Trued BC ?

[lte82]

I shoot a lot at sea level with DA's in the 0 range, and have trued my BC's to work well here, but found this weekend that I had to tweak my BC's of both my 6.5 and 6x47 up like 3-5% to make them work at a higher DA (1500-2000 I believe). With the 6x47, using the trued G7 I use in the swampy areas I typically shoot inside 500 yards it was still fine, but by 700-800 I was about .1 mil high, and at 1050-1070 was like .2-.3 mil high. So I guess my question is, is there a formula or maybe a rule of thumb for in/decreasing BC based on DA, or just something you have to take good notes on? Or maybe this is all shooter error? If it had only been one gun I might have written it off, but both guns needed to come up on BC a good bit.

Edit: Using a Kestrel w/ Applied Ballistics

 

[_Windrider_]

I shoot a lot at sea level with DA's in the 0 range, and have trued my BC's to work well here, but found this weekend that I had to tweak my BC's of both my 6.5 and 6x47 up like 3-5% to make them work at a higher DA (1500-2000 I believe). With the 6x47, using the trued G7 I use in the swampy areas I typically shoot inside 500 yards it was still fine, but by 700-800 I was about .1 mil high, and at 1050-1070 was like .2-.3 mil high. So I guess my question is, is there a formula or maybe a rule of thumb for in/decreasing BC based on DA, or just something you have to take good notes on? Or maybe this is all shooter error? If it had only been one gun I might have written it off, but both guns needed to come up on BC a good bit.

Edit: Using a Kestrel w/ Applied Ballistics

Well more than likley what happened is your MV was different due to the higher DA due to a temp change. BC is velocity dependent more than altitude dependent and temp changes velocity, temp also changes DA. give me some atmospherics and i can run through AB Analytics and see what it shows.

 

[lte82]

Well more than likley what happened is your MV was different due to the higher DA due to a temp change. BC is velocity dependent more than altitude dependent and temp changes velocity, temp also changes DA. give me some atmospherics and i can run through AB Analytics and see what it shows.

Ok awesome, thanks. MV's were correct in the kestrel, I checked before I shot. 108 Berger trued G7 at 0 DA was .264 (3070 fps MV), and at 1800 DA it was .280 (3100 fps MV, barrel sped up some due to finally breaking in, which probably pushed the BC up a little too maybe). Also a 147 eld trued up to .330 @ 0 DA, and .345 @ 1800 DA, both right at 2780 fps. Actual elevation change was about 900 ft between locations.

 

[_Windrider_]

Ok awesome, thanks. MV's were correct in the kestrel, I checked before I shot. 108 Berger trued G7 at 0 DA was .264 (3070 fps MV), and at 1800 DA it was .280 (3100 fps MV, barrel sped up some, which probably pushed the BC up a little too maybe). Also a 147 eld trued up to .330 @ 0 DA, and .345 @ 1800 DA, both right at 2780 fps. Actual elevation change was about 900 ft between locations.

Well there is your answer....it was a MV increase likley due to higher temps. it has nothing to do with DA. DA will affect the drops over the range of shooting, but velocity and BC or directly proportional. MV goes up so does BC and vice versa. the published BC is only valid at the given MV, which is also always measure at ISA (29.92 inHg, 59F and 0RH which coincidentally equals 0 DA).