How real world BC vs Published - how far off can it be?

[Mr_Happyface]

Gents,

Working up a load for a 7mm rifle. I'm shooting the Berger 190 Hybrid. The published BC for this projectile is .751

I got the load I was looking for and my muzzle velocity is around 3060. I zeroed and took it to 1K and was doing good at first, but the further out I shot, the more I would miss high. At 1000 I was .2 mil (or greater) less than predicted (IE computer told me 6.2, real world was 6.0).

I was running a labradar the whole time and looked at the velocities on the shots that registered out to 100. Plugging those numbers into Field Firing Solution I found I needed a BC of .810 to make the Muzzle and 100Y velocities match what the Labradar showed. This was consistant across the 10 or so rounds that registered.

The conditions were roughly 29.00, 66deg, 40%, slight left crosswind, shooting upslope, but FFS takes that into account - it's flat to 100y.


My question is:
I know BC is dependent on velocity and 3060 is hauling for a 7mm - is it possible I am actually achieving .810 with my long-ass barrel?

Can the difference be as great as 0.059?

If not, should I tweak the DK in FFS or the BC?



Thanks. Also, @Lowlight - I'm loving the LH gain twist barrel. Don't know how to describe it, but the recoil is just 'better'

 

[CoryT]

.059 would be a pretty large shift. Couple of things to question. Has the scope been properly calibrated? Was the 1000 yard group shot on paper, 10 or more rounds and measured with a ruler, or shot on an electronic target so we have the impact point and velocity? Just shooting steel and estimating POI? How were range and angle determined and entered?

Also, for this cartridge you are working at too short a range to reliably compute DK, and really even BC. 4/5 of supersonic is the sweet spot, get to where the bullet is running at 1200fps or so.

 

[Mr_Happyface]

Scope is calibrated. 1K was shot on fresh painted steel and I used the .1 hashmarks to see the amount I was missing by. Range was laser ranged (was really 1010), angle was with rifle mounted indicator, (8 deg).

Ok, next time I go out I'll get some paper up there and see what happens.

The main thing that makes this confusing is that labradar velocities predict a BC that matches real world performance. I don't get it.

 

[Hula Popper]

Any reason why you’re using a G1 bc? G7 may give better results, especially at that range.

 

[Mr_Happyface]

FFS only works with G1

 

[6.5SH]

Think you'd need an Oehler 89 or shotmarkers at distance for anything truly meaningful.
But if your LabRadar had good tracking I'd be curious what this showed:

Geladen | Labrabaco