BC's vs velocity?

[midwesthunter]

I have some questions about BC values and how velocity changes them. Lets take the 300 SMK for example. specs are .768 BC at 2300+fps. then reducing to .760 at 2300-1800 down to .750 at 1800 and below. Shouldn't the BC be higher at increased velocities? Say the 3000fps mark? I read somewhere and cant find it again so maybe I read wrong but from what I recall BC value would drop about .02 every 100fps. Is this true or is it more on bullet design/shape?

 

[rscott5028]

Hopefully, Bryan Litz will jump in and answer your question. I believe he's written books on the subject.

In any case, it's my understanding that the G7 projectile more closely represents modern bullets and therefore more closely models the flight characteristics throughout each flight regime. Hence, one good G7 BC will more accurately represent your flight path rather than multiple G1's.

As for 2300+ being the max G1 BC bracket listed for a particular bullet, I would hypothesize that you might get a slight improvement as you go up in velocity. But, with such diminishing returns as to be not practical nor worth measuring using modern cartridges and velocities. And, if you were only speculating, then it'd be easier to forecast using the G7.

...but, I'm keen to hear from the experts.

-- richard

 

[MontanaRifleman]

G1 BC's usually improve with greater velocity with low drag bullets because G! BC's were not developed for low drag bullets. G7 BC's which are developed for low drag bullets remain constant at all velocities.

Hope that helps,

-Mark

 

[load]

higher speed = higher resistance.
one problem u run into when shooting high bc bullets at high speeds is the appearance of a secondary shock wave at the shoulder of the bullet. the sharper the transition from ogive to bearing the worse the problem. this is one reason (besides most bullets dont fit the g1 curve anyway) sierra give multiple b.c.s' it is also why you see "hybreds" popping up instead of pure secant ogives. this problem is worse at sea level than at the mountain tops and can come and go accourding to barometric pressures causing sudden drops in bc and really throwing off a shot. if you are shooting high vld shapes keeping the velocity below 3000 fps (possibly even 2800 at sea level) can really help with this problem. this is most pronounced at shapes over 14 calibers of ogive but can be found even lower