Why do the bc numbers hate 22 cal bullets?

[coyotekilla]

So, I'm no balastician, or math guru even, but I've noticed some inequalities in bc calculations.

An 88eldm or 90 gr berger. 224 bullet have a g7 bc of .274 yet require a 1:7 twist to stabilize.

A 245gr 30 cal berger is happy with a 1:8 twist. G7bc of .433

Why do bullets of simular shape, simular twist needed for stability have such different bc? Is there a math anomaly that hates little bullets?

 

[alexnelon]

Ballistic coefficient is a measure of drag resistance, not bullet shape.

Bullet weight and velocity have more impact on external ballistics than bullet shape, generally speaking.

 

[wpeach1912]

I could go do the math, but I think it has to do with just density. Mainly you increase volume of a cylinder (the shape of a bullet for this discussion) very quickly as you increase diameter which quickly leads to a higher total mass. I think that extra mass helps give better stability at lower RPM.

 

[AkMtnRunner]

As size increases, surface area increases exponentially to the second degree while volume (mass) increases exponentially to the third degree.

Think of a pin needle free falling vs a large rod of same metal and shape. There’s much more inertia for the amount of drag with larger objects, all else being equal.

 

[coyotekilla]

I could go do the math, but I think it has to do with just density. Mainly you increase volume of a cylinder (the shape of a bullet for this discussion) very quickly as you increase diameter which quickly leads to a higher total mass. I think that extra mass helps give better stability at lower RPM.

I thought that might be the case but the elr guys shoot machined solid copper vs lead core bullets. Now it might be helpful to see the balistics of d.u. rounds but I'm not military anymore, nor are coyotes and paper considered high value targets.

 

[AkMtnRunner]

I thought that might be the case but the elr guys shoot machined solid copper vs lead core bullets. Now it might be helpful to see the balistics of d.u. rounds but I'm not military anymore, nor are coyotes and paper considered high value targets.

They use machined solid copper because that ends up being more consistent than lead core bullets. Even if it’s a worse BC, it’s more predictable.

 

[nm.otter]

COAL limitations, chamberings and the like (even the newest ones) were built around and are optimized for cup and core projectiles.

If UM succeeds in their quest for super-heavy 6mm slugs optimized for magnum (and UM) cartridges, applying those profiles to less-densely constructed bullets could get interesting.

 

[coyotekilla]

COAL limitations, chamberings and the like (even the newest ones) were built around and are optimized for cup and core projectiles.

If UM succeeds in their quest for super-heavy 6mm slugs optimized for magnum (and UM) cartridges, applying those profiles to less-densely constructed bullets could get interesting.

Um? Ultra magnum?

 

[Mike Islander]

High BC bullets have high ratios of mass versus diameter. For a 22 to match a 6.5, it needs to be much longer than standard, like a 1.25" to 1.5" long bullet. And it's difficult/impossible to stabilize a 22 bullet that long.

 

[Bluefish]

I had never given it much thought, but having worked on aero drag for other projects, there are two sources of drag, frontal area and skin friction. So for any weight there is an optimal length/ diameter where both are minimized. Thus you can’t reduce the drag of a 22 cal by making it as long as a 30 cal.

Not sure if the Re also come into play. It’s related to length, so longer bullets will have higher Re numbers as length is in the numerator.

 

[Mike Islander]

Skin friction is one of the least important types of drag in ballistics. Form drag, base drag, and wave drag account for about 90% - 95% of total drag. Low mass versus diameter dooms the .224 bullet to lower usable BCs.

 

[cal30_sniper]

I had never given it much thought, but having worked on aero drag for other projects, there are two sources of drag, frontal area and skin friction. So for any weight there is an optimal length/ diameter where both are minimized. Thus you can’t reduce the drag of a 22 cal by making it as long as a 30 cal.

Not sure if the Re also come into play. It’s related to length, so longer bullets will have higher Re numbers as length is in the numerator.

Reynolds was where my head went.

 

[TaperPin]

I have no clue, but like the wide range of ideas. I always assumed it was less spinning inertia in little bullets so the extra spin is to bring the inertia up to similar levels, but that might be totally off base.

 

[coyotekilla]

I have no clue, but like the wide range of ideas. I always assumed it was less spinning inertia in little bullets so the extra spin is to bring the inertia up to similar levels, but that might be totally off base.

Valid theory!

 

[Okie_Poke]

Um? Ultra magnum?

Unknown Munitions, I believe.

 

[nm.otter]

Both, actually.