Bullet bases and aerodynamics

[craig110]

Since the "teach me about bolts" thread quickly answered my nagging question about what makes one bolt better than another, here is another nagging question I've had: Why do bullets generally have flat bases? (And for purposes of this question, boat-tail and RBT bullets still count as generally being a flat base.) The reason I question this is that the profile of other items designed to travel supersonically - such as the wings and fuselage of airplanes - do not abruptly end in a flat back-side but rather gently taper off. Conceptually, the flat back-side would seem to produce a low-pressure area that would be a drag on the object while a smooth taper gives a greater chance of a nice laminar airflow.

Bullets with tapered noses have larger BCs, so why are long-range bullets not shaped more like a double-ended bullet with a taper on both ends? I'm sure the rear taper wouldn't be as many ogives as the front just due to taking up case volume, but it seems that even a rounded base would give better aerodynamics than would even an RBT. So, since bullets tend to have flat bases (again, counting BT and RBT in this category since their flat areas are just a bit smaller), what am I missing?

[MONTEGOD7SS]

I am going to guess that it is based more off internal ballistics than external ballistics. Being aerodynamic doesn't matter if you can't get it out of the barrel consistently and this is why I think they are flat. By being flat(ish) it gives the gas more area to push straight on versus pushing out if a bullet had a tapered rear section. Koski should chime in here soon with the super-deadly rear-facing SWC, but most people can't handle something that dangerous.

[craig110]

Hmmm, true. Even a round-nosed base would tend to deflect the internal pressure wave towards the side of the bullet instead of just pushing it forwards. I was thinking too much about the bullet in flight and not the launching of it.

[918v]

Bullets need a "shank" about one caliber long to be stable in the barrel. If bullets were shaped like airplane wings, they would be way too heavy. Their present shape maximizes case capacity, ensures in-barrel stability, and still allows for efficient flight.

[JLarsson]

I'll bet production has something to do with it as well. Some of the VLD bullets have pretty streamlined bases yet are flat on the bottom. Easier to swage a bullet with a flat base or at least a flat ON the base.

I wonder also how much of it has to do with the fact that the bore-riding portion of the bullet has to be cylindrical. At some point behind that, any further taper or shaping may be meaningless for a projectile that's rotating at maybe over 200,000 rpm. Or not - just thinking out loud here.

[MakeMineaP99]

Craig,

Did you ever buy Understanding Firearms Ballistics, Rinker, Robert A? This will answer all your questions and many more.

I'll let you borrow my copy if you want it, I won't need it for a while. I have a reference for just about everything.

[craig110]

Thanks for reminding me about that book. I just checked on Amazon, and it is cheap enough ($16 and change) that I'll just order a copy this evening instead of us paying the good folks at the post office to haul it back and forth. Thanks for the offer!