Ballistics Question: Bullet shape

[jamz]

Why do most bullets have a flat or truncated aft section? Why isn't the trailing part of a bullet shaped more pointedly, like a submarine? I would imagine that it would make it that much faster, achieving higher velocity along it's flight.

Only think I'm not sure of is how bowder burning and expanding gasses would affect a conical shape instead of a flat one.

Anyone have any ideas on this?

-James

 

[Jim March]

Four reasons:

1) Bullets are stabilized by the rifling. The more bullet rubs up against said rifling, the better the accuracy.

2) You want less of that hot gas to blow past the bullet through the grooves, therefore you do a flat base. If you tried to use a "double pointed spitzer" the rear point would "direct" some of those gasses around the bullet and muzzle velocity would suffer.

3) There's also the issue of case volume. The "rear point" would take away too much powder area, at least in some cases.

4) Terminal effects might suffer in some instances - certainly at speeds below 2,200ish. (That's the approximate speed where hydrostatic shock starts.)

There have been some "boat tailed" rounds that start to go this way...but they're usually somewhat small bullets fired from large cases (see also the more exotic varmint calibers/wildcats) and their terminal effects come via extreme fragmentation so the shape is more or less irrelevent to the wounding characteristics on prarie dogs or whatever (which simply blow up).

 

[Zak Smith]

The base if the bullet is much more critical to accuracy than the front end of the bullet. In other words, bullets which have more consistent base construction will fire more consistently than those with defects.

Match bullets are most commonly open tip because the bullet is contructed using a copper jacket stretched over the base. This provides an extremely consistent base surface and geometry.

Furthermore, boat-tail bullets are not the best for accuracy! I believe most benchrest shooters use flat-base bullets to minimize disruption as the bullet leaves the rifling at the muzzle. The boat-tail is primarily a helper for ballistic coefficient (BC).

-z

 

[Zak Smith]

Alternatively, if you put the center of mass ahead of the center of drag, you no longer need spin stabilization. This kind of a projectile would be more like a fin-stabilized dart you see in APFSDS rounds from tank canons.

-z

 

[griz]

They might require a faster twist to stabilize because of their length. Also they would be difficult to manufacter. Interesting idea though. I think the closest thing to a pointed base bullet available are the 80+ grain 223 bullets they use in high-power matches. The adavntages wouldn't show up until way out there.

 

[Zak Smith]

The rate of twist required for stable flight is a function of the torque applied to the front tip of the bullet about the base by the atmosphere.

The torque is a function of the force (pressure of the wind) multiplied by the fulcrum radius -- which in this case is the distance from the forward center of drag to the center of mass.

I would think if the fulcrum radius were decreased, e.g. by having a shorter ogive and a longer tail, that it would require less spin to be stable.

-z

 

[griz]

I may be stepping into the deep end when I should be in the kiddie pool, but I would think a longer tail would move the center of mass further back than any offseting effect out front. Also, assuming we are talking rifle bullets, the thing should stay super sonic so you don't want to blunt the nose any more than you have to.

 

[Zak Smith]

If a bullet was symmetrical forward & back -- if it looked like two cones connected to a center cylinder -- the cemter of mass would be in the middle.

A conventional bullet's center of mass is pretty far to the rear. I'd guess only about 25% forward of the back of the bullet.

I was trying to say that the distance from the tip to the center of mass would be longer in a conventional bullet. I'm not sure it's true, but it seems geometrically sound.

-z

 

[redneck2]

You guys think too much...

I suspect the real answer is that, as above, accuracy is determined by the base as much as anything

if the muzzle crown is dented or distorted, it lets the gasses escape out the notch and blow the base of the bullet off to the side disrupting the flight

A sloped base would make this far worse. Gasses would accelerate up the ramp made by the base of the bullet and really blow it off target

I suspect best accuracy would be a totally concentric muzzle and totally flat base bullet

I do know that, in long range black powder cartridge shooting, a totally flat and square bullet base is considered critical. That's the reason that most cast bullets have the sprue at the front

 

[Frohickey]

Flat base would allow for more powder capacity per bullet weight
Flat base would allow less gass blowback (correct term) that would affect velocity.
Flat base is easier to construct (consistently)

Some manufacturers have made boattail bullets, and these are longer per bullet weight, affecting case capacity. Also, boattail bullets can be susceptible to gasses from the rear rushing up to the front, as it exits the barrel, and this could affect accuracy.

Boattail bullets do help out in retained velocity AT LONG DISTANCES. So, some manufacturers have made rebated boattail bullets.

 

[ceetee]

What if you took your bullet, complete with "over-sized boat tail", and planted it in a short sabot?

 

[Majic]

That's the reason that most cast bullets have the sprue at the front

The vast majority of bullet molds are not nose pour designs and the sprues are on the base.

Most rifle target bullets are a hollow point design for the purpose of shifting the center of gravity toward the rear of the bullet.

 

[Jim March]

Yes, a sabot might help. And if you didn't care about "case size efficiency" you'd be in good shape (big case needed because the oversize sabot is going to eat case capacity).

Example: 223 double-pointed critter in a 30-cal sabot and a 30-06 case.

You could make test specimens out of pure copper in a lathe, by hand...

 

[Slimjim]

Rainier bullets has match pistol bullets which are not flat on the bottom, but have a flat circle then the center is domed. But the dome doesnt go al the way to the edge of the bullet though.

 

[Majic]

Rainier bullets has match pistol bullets which are not flat on the bottom, but have a flat circle then the center is domed. But the dome doesnt go al the way to the edge of the bullet though.

Sounds like a variation of a hollow base with a thicker skirt to handle higher pressures allowing higher velocities.

 

[ceetee]

You could make test specimens out of pure copper in a lathe, by hand...

N-a-a-a-a-h-h-h!


You could make test specimens, maybe... I don't even own a lathe!