Two exactly identical bullets or what?

[no 23]

Lets say that we have 2 exactly identical bullets that have the exactly same form on the ogive and the boat-tail they are both 155 grs and .30 cal. But one bullet is a litle longer than the other.

Bullet A Is longer on the straight part of the bullet than bullet B . This is the only difference. Same weight and same form, apart from bullet A being longer on the straight part.

Which one of the bullets has higher BC, bullet A (the longer one) or bullet B.

Person A: argues that the resistance of the air is higher on Bullet A because it's exposed to more air (longer body) as it travels through the air, there for the BC must be lower on the longer bullet.

Person B: argues that the longer bullet (Bullet A) has better (lower) form factor there for it must have higher BC. At least it should fly better of the two.

Can you solve this?

 

[Bryan Litz Ballistics]

A is correct.

More surface area = higher skin friction drag, lower BC. Especially when the extra surface is engraved with riflings.

The difference wouldn't be great, as most of the drag is made up of wave (form) drag and base drag. Skin friction is the smallest component of overall drag but A definitely has more skin friction drag than B.

-Bryan

 

[no 23]

Thank you Bryan!

This makes sense, so i'll have to bow to my mates argument. But you talk about 3 diffrent drags! Skin friction drag (when the bullet rubbs against the air on it's flight path) and then wave form and base drag, is there any chance that the longer bullet outpreforms the shorter in both these areas and there for flies better than the short one?

Please say so... i'd hate to admit i was wrong...

 

[JohnKielly]

Nope. It could be achieved with marginally different core weights, which would then impinge on how far forward the core came in the jacket.

 

[no 23]

That is correct gstaylorg. This is an unrealistic scenario. This was just a pure debate about whether the friction on the side of the bullet would be something that would matter during the flight path or not. I was convinced that the form factor would be lower on the longer bullet, and there for it would fly better.

Since then i did a little reading on the term form factor again and realized that the straight part of the bullet doesn't matter in form factor calculations, only the shape of the ogive, meplat diameter and the boat-tail.

Sectional Density: Is found by dividing weight with 7000 (to convert from grains to punds) and then dividing the outcome with cal x cal. Looks something like this. (Weight/7000)/(cal^2). For these particular bullets the Sectional Density is the same. (155/7000)/(.308^2). And i have a pocket calculator to calculate this -> 0,0221428571428571 / 0,094864 = 0,233

G7 Form Factor: Is found by dividing a measured form factor by the G7 standard

G7 BC of a bullet: Is the SD / Form Factor.

Side note (lesson learned): Note that the length of the bearing surface is unimportant to the bullets drag and form factor. Only the nose length, nose profile, meplat diameter, boat tail angle and boat tail length dictate what the drag and form factor of the bullet will be.

Thanks again Bryan for taking a time answer my question...

P.s. this should be achievable with 1 bullet made of copper alone and the other made of copper with lead core, but that was not the question whether this is something that exists or not, just a theoretical question.

 

[JPeelen]

Bryans answer is of course correct. But my impression is your conclusions should consider other factors also.

Looking at bullet geometry alone can be misleading. A bullet is a flying top that rotates extremely fast. It always is subject to some form of conig motion, which is influenced by its dynamic properties. Adding a lead core to a copper bullet not only changes overall mass, but also its moments of inertia. This changes its flight behaviour and consequently the effective drag.

From what I have seen, I doubt that it is possible to predict which of the two very similar bullet shapes will show larger overall drag when fired on a real range. Results may even vary with the actual barrel used.

In short: do not look at geometric shape alone. For example, the effect of a given boattail is influenced by the bullet shape ahead of the boattail. Even today's most sophisticated aerodynamic computer programs can only yield approximate results. There is no real alternative to making a bullet of the desired design and testing it on the range for getting a reliable answer.

 

[GotRDid]

Swage forming a core of tungsten/bismuth (not unlike the non-toxic waterfowl shot) alloy to very precise tolerances wouldn't pose too much of a problem for the most modern bullet manufacturers in spite of the cost per unit. Along with some uber precise jackets, and the attention to detail in completing the forming of the finished bullet and you could indeed customize the "windscreen" around the core. It would likely be a very short core relative to the traditional lead putting the weight bias a bit further to the rear of said bullet. Advantages? I have no idea.
Am I thinking clearly in this regard?
GotRDid.

 

[Endyo]

As I was reading Bryan's response I was thinking, man this guy knows what he's talking about. Then I saw his tag and was like, "Oh...well yeah."