Range Report 69gr SMK what am I missing?

[RG in Idaho]

I've searched for an answer but everyone seems to accept the following;

69smk bc's from Sierra's site:

.301 @ 2800 fps and above
.305 between 2800 and 2200 fps
.317 @ 2200 fps and below


The BC improves as velocity decreases. Is this correct???

Every other bullet in the line shows deterioration in BC as velocity goes down.

What am I missing?

I thought I saw something from Litz indicating an average BC of .331 for the 69smk.

 

[TimK]

I just looked it up on Ballistic FTE, and it shows .301, .305, .301. Different, but just as odd.

 

[Early Cuyler]

I've searched for an answer but everyone seems to accept the following;

69smk bc's from Sierra's site:

.301 @ 2800 fps and above
.305 between 2800 and 2200 fps
.317 @ 2200 fps and below


The BC improves as velocity decreases. Is this correct???

Every other bullet in the line shows deterioration in BC as velocity goes down.

What am I missing?

I thought I saw something from Litz indicating an average BC of .331 for the 69smk.

I asked this same question about the .224 90gr SMK (BC is listed at .504 over 2200fps and .511 between 1900-2200fps) a while back here and emailed Sierra about it. This is what I got in an email from Sierra:

Good question and No, it isn’t a misprint.

Air is a lot like water at high velocity so the faster the bullet the greater the pressure wave in front of the bullet which lowers the efficiency of the bullets shape. At lower speeds the design is more efficient and flows through the air but at higher speeds the air “piles” up in front of the bullet reducing its efficiency.


Rich Machholz

Ballistic Technician

 

[jbailey]

BC is a relative factor, i.e. relative to a model projectile (actually form factor is the relative factor, but don't worry about that now). Scientists know how this model projectile retains velocity. So the BC relates your bullet to this model projectile and allows you to use those ballistic programs to solve for real world firing solutions. Some bullets will be more efficient compared to the model projectile at certain speeds and less efficient at other speeds, which is why BC changes with velocity change. Typically, this relative difference is not important enough to have a real impact on your ballistic solution, so it is safe to use an average BC across a wide velocity range and not worry about the small changes to BC at difference velocity bands.

Now if you were to look at drag coefficients, they all spike as the bullet slows towards the sound barrier. That is what you are thinking about with your "shows deterioration" thought.

 

[RG in Idaho]

Thanks for the insight

 

[Joe-n-TX]

I like Sierra. Stats are one thing. Reality is another. Leave the numbers in the books and and shoot to find out what is actually the best. There are way more variables other than the bullet itself.

 

[damoncali]

If your BC changes with velocity, it means you're using a poor drag function- varying the BC is nothing but a mathematical kludge to make up for that fact. I love Sierra's book - it's one of the best in my opinion - but they really need to embrace the modern age with this stuff.

You'll find that the G7 drag function is a better fit for that bullet, resulting in a G7 BC that remains much closer to constant relative to velocity. You can find it in Bryan's book.

 

[TiroFijo]

In this case, it obviously tracks the G1 curve very well

I doubt you'll see less variation compared to the G7 curve. At least in this case it won't matter until you get close to the subsonic range.