Weight a factor of BC?

[8andbait]

I answered a question that wasn't asked.

That's my bad for skimming too quickly, but let's try to remember we are all in the same team.

 

[ndking1126]

My conclusion - weight/mass is a factor in what the BC actually is - however - for two different weight bullets with the same BC, their drop will be equal if fired at the same velocity.

Somebody correct me if my logic is wrong?

Your conclusion is spot on, to the best of my understanding.

 

[jebel]

Ballistic coefficient - Wikipedia

en.wikipedia.org

I did NOT model it to determine the impact of weight on BC ... feel free to and share it back

If I'm reading this formula from Wikipedia right, weight directly affects BC. If you double the weight (maybe a tungsten bullet instead of lead), holding all else constant, you double the BC.

 

[Tiny Tim]

You guys made me think (ouch) ...

I think we have a bit of a "both right" scenario going on here ...


Hypothesis to be proven or challenged:
If the BC of 2 different weight projectiles is the same - their flight characteristics are the same regardless of their weight (i.e. same drop at 1000 yards)

Here is the (simplified) equation used to calculate a bullet's BC (based on the wiki page):



BC(projectile) = Mass(projectile) / [Diameter(projectile) ^2 * i(coefficient of form)]


So, that which makes up the BC for those projectiles is in fact a factor of Mass - as well as Diameter and their coefficients of form ...

So, from what I am understanding (so far) on this - 105g 6mm w/ BC of .6 (making this up) and a 140g 6.5mm w/ BC of .6 (making this up) should both exhibit same drop across their path as long as their muzzle velocity is the same ...


So to see if this is correct, I plugged these two into the Hornady G1 calculator ... both with .6 G1 BC ... both at 3000fps muzzle ... leaving everything else the same and only varying the weight and ...

Drum roll ...

... Both have 7.2 mils drop to 1000 - spot on the same

My conclusion - weight/mass is a factor in what the BC actually is - however - for two different weight bullets with the same BC, their drop will be equal if fired at the same velocity.

Somebody correct me if my logic is wrong?

You are correct, yet this is a scaling issue. If the bullets are same size and form but different materials, the heavier will have the higher BC.

 

[nkyshooter]

You are correct, yet this is a scaling issue. If the bullets are same size and form but different materials, the heavier will have the higher BC.

100% correct ... the "size and form" are what make up the "Diameter(projectile) ^2 * i(coefficient of form)" in the denominator of the equation.

This tells me that this is why the mono (all copper) bullets - which are of lesser density - must necessarily be made to minimize the product of the square of the diameter and the coefficient of form ... which means they must lengthen significantly to be equal in weight to jacketed lead ... which means higher twists required to drive bullets of same weight as compared to a jacketed lead projectile in the same diameter.

 

[seattleman1969]

Maths.... headaches

 

[BrentM]

All a person needs to do is plug in to the BC formula. Weight is always a factor but not nearly as much as some believe. For example, doubling the weight doesn't double BC. Weight is just one part of the whole. Clearly in the example you are using a dimensionally same bullet and only changing the weight.

Myth- higher weight mean higher bc. Not true by itself. Think round ball, round nose, short stubby fat bullets.
Myth- smaller caliber and lighter bullet is more effected by wind then large heavier caliber. Not true. It is directly related to BC.

On another note, it has been proven than BC is affected by velocity so in some cases a lighter, lower bc, at a higher velocity can have better ballistics then a slower, heavier, higher BC bullet. Generally the terminal ballistics is all that is negatively effected and that is not an always the case gig either.

Something I always thought was strange is how a 143 eldx has less bc than a 140 eld. Even a 135 a has higher BC. When breaking down the bullet the form factor etc the bullet shape and design, even though very similar tends to net slight differences in bc.

 

[264MHC]

Theoretically two bullets that have the same BC fired at the same muzzle velocity should fly the same. But I have read where people with a lot of experience claim that the heavier bullet will do better in varying conditions because of its larger mass and momentum. For example, a .400 g7 300 Grain .338 bullet vs a .400 g7 190 Grain 7mm bullet, people claim that the 338 bullet will fly better even though the BC is the same. I think that is part of the reason ELR shooters use .375s, .408s, .416s and such even though smaller calibers may be able to match them in Theoretical ballistics. I'm just passing on what I've read but don't have enough experience to say for sure so take it for what it's worth.

 

[BrentM]

Theoretically two bullets that have the same BC fired at the same muzzle velocity should fly the same. But I have read where people with a lot of experience claim that the heavier bullet will do better in varying conditions because of its larger mass and momentum. For example, a .400 g7 300 Grain .338 bullet vs a .400 g7 190 Grain 7mm bullet, people claim that the 338 bullet will fly better even though the BC is the same. I think that is part of the reason ELR shooters use .375s, .408s, .416s and such even though smaller calibers may be able to match them in Theoretical ballistics. I'm just passing on what I've read but don't have enough experience to say for sure so take it for what it's worth.

Tail whip is much more pronounced in smaller lighter bullets for sure. I experience that shooting 6mm vs 6.5 at PRS matches. It was an interesting education. It is also interesting that the 1000 yard records are still held by small caliber.

 

[Mike Matteson]

I will go along to a point. 30 to 40 years ago. I did some reading on velocities in different rifles. If they were at the same velocity the drop of the bullet is about the same if not right on. The heaver bullet will have more fpe than the lighter bullet. Wind drift is a factor too. The shape of the bullet play into it. That is why I stayed with rifles the could product about the same velocity. More of my rifles do hit the same velocity. Weather I am shooting a 25-06, 264 Wm, 7mm Rem M, 308 Norma M, or 338 W Mag. I really think that you should be looking Foot Pounds of Energy at those range to see if it's going to do the job.

 

[Calamity]

Hi Guys. The thing that is missing in this thread so far is inertia. If two bullets have the same shape and dimensions, but one is heavier, the heavier bullet will resist change more than the other. That is inertia. The one with the greatest inertia will resist a change in conditions longer. It will resist a change in velocity longer, and change in spin, a change in potential kinetic impact energy, and therefore, given the same starting velocity, it will sustain velocity longer, meaning extended range, greater impact energy, less wind drift, etc.

 

[Calamity]

For more info: https://en.wikipedia.org/wiki/Inertia

 

[TBell]

If you take any given round, let's say .308 in 150 grain, and make it heavier to 165 grain by making it longer, is the extra weight or the extra length what is responsible for the increased BC?

Length

 

[Calamity]

Just for a fun experiment, buy a kid's hula hoop (hollow rigid plastic toy) and cut it in half. Obtain two metal balls of the same size (Walmart sporting goods) that will roll through the hollow hoop. One ball should be steel (ball bearing/slingshot ammo), the other lead (fishing sinker). Mount the half-hoop on a piece of plywood with both open hoop ends pointing up. Drop each metal ball into one end of the hoop and mark how high it rises in the other end. The heavier ball will rise higher, retain velocity longer, etc. Teach your kid about inertia, ballistic coefficient, etc. Make a memory.......