Range Report how can this be?

[shootist2004]

sierra MK 120 gr BC .42@3100vel
sierra MK 123 gr BC .51@1635vel

higher bc at half the vel. I thought it was a typo so I called them and they said it was correct. Who would buy the 120?

 

[MontanaMarine]

Re: how can this be?

Probably significant differences in the ogive, and boat-tail.

 

[BallCoeff_606]

Re: how can this be?

For the same bullet, you can even have 3 different BC's listed, one for each velocity range. BC probably changes constantly, depending on velocity, but manufacturers just give one (or a very few) that are most representative.

So, operating velocity has as much to do with BC as bullet shape. My guess it's the transsonic boundary which has the biggest effect on BC for any particular bullet shape.

 

[x832]

Re: how can this be?

Also Sierra gives BC ranges so one can determine what shape of bullet has the best BC for a certain powder load.For instance in determinig which bullet is going to drop less and less spin-drift minimized-not eliminated.

 

[AshesToAshes]

Re: how can this be?

I've read that rounder nose bullets travel better at slower speeds, which is why most of the subsonic ammo you see factory loaded use round nose. Even though these BCs are slightly above subsonic, judging by the low velocities Sierra advertise, maybe they were designed to fly at the slower speeds. So the 123gr might not fare as well at 3100fps. But more the likely Sierra is known for having some oddball bullets that just don't fly well. The .284 160gr HPBT GK is one of them (they opened up the nose) and unfortunately I have a half dozen unopened boxes of the stuff that I'll never use.