I was looking at Lost River Bullets and got to wondering what the best bullet shape might be. Even the best boatails have a flat base. People have loaded bullets backwards before, do they lose velocity? Say I was looking into a 6.5 WSM in a 24-26" barrel, a pointed rear would take up powder space making the cartridge less overbore. A plus in this case.



Help, I can't stop my brain. <img src="/ubbthreads/images/graemlins/tongue.gif" alt="" />



Another thing, are those high BCs that Lost River states reliable?

Interesting thought ... I am curious to hear the results of this inquiry.

I like the idea of a football shaped bullet .... Hey Mr. Elk, I want you to runout 75 yards and cut to the left and stop angling away ... the ball will be there when you stop ... don't drop it!! <img src="/ubbthreads/images/graemlins/grin.gif" alt="" /> <img src="/ubbthreads/images/graemlins/grin.gif" alt="" /> <img src="/ubbthreads/images/graemlins/grin.gif" alt="" />

Because they'd roll off the bench if you couldn't stand them up.

Think about it.

Go back to the inescapable basics.

Inside the barrel, the expanding powder gas needs a surface to push against with as much force as possible.

Outside the barrel, the bullet needs to cleave the air with as little resistance as possible.

Ergo, a flat base to push against, a pointy tip to cleave the air.

Only Capstick and his buddies <img src="/ubbthreads/images/graemlins/grin.gif" alt="" />.

Chuck

I'd also suppose that a pointy tail would induce turbulence as the airflow came back together at the "base". (Of course racecar aerodynamics may not apply equally to boolitz)... <img src="/ubbthreads/images/graemlins/tongue.gif" alt="" />

JimF

Because that would confuse some of us, we would not know which end is which. <img src="/ubbthreads/images/graemlins/grin.gif" alt="" />

I did think about it. And it is a function of differential pressure. Assuming the bullets path is in the x-direction, the amount of surface area that pressure acts upon in the x-direction is the same whether the bullet has a flat base or if the bullet has a conical base. For a .308 bullet, a flat base bullet has a surface area of .0745" squared. A boat-tail or conical tailed bullet had signifcantly more surface area in the rear of the bullet but the surface area that the pressure can act upon in the x-direction is still the same .0745" squared.

At a guess, I'd say that a pointy rear end would cause a laminar flow along the bullet while still in the barrel. If so, the gas would hug the bullet and flow around it, breaking up into vorticies in front. This would likely cause it to be very inaccurate. I couldn't guess what kind of velocity you'd get.

You might, however, use a sabot/shoe at the rear to break up the laminar flow and which would fall off after exiting the barrel, as tank rounds do. You'd then, theoretically, have a bullet pointed at both ends flying very efficiently through the air. That would certainly be a rather extreme boattail.

Guessing.


Jaywalker

Jaywalker,



I'm not sure I can agree with you. If the gas can create a laminar barrier around the "football" bullet, it can do it with any bullet, especially a boattail bullet. (At those pressures and velocity I don't see very much laminar flow whatsoever except for perhaps the very few monolayers of gas next to the barrel's wall.) IMO

George,

Could be. In order for my theory to be valid, you'd have to assume that a "normal" boattail base would create vorticies that disrupt the laminar flow.

Jaywalker

I believe JimF is on the right track. I think the rear tip would increase drag, particularly if the bullet was symmetrical front and rear. I don't have the fluid mechanics books in front of me to prove my theory, but I'll see if I can dig up something in the next few days. Perhaps someone else can answer it definitively before I have to dig into some research on the subject. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />

Net cross-sectional area isn't the point. You don't want a pointed base cleaving the gas column and further encouraging flow-by and gas-cutting, which can still be bad enough with flat-base bullets.

Shouldn't that be 0.154 squared times pi?

Re laminar flow:

A very common error is thinking of the powder-gas flow as like the unidirectional flow of a given volume of another gas (or a liquid) pumped through a tube -- as opposed to a rapidly and violently expanding gas that's forcing its own way free by pushing hard against every resisting surface. For one thing, gas pumped through a tube by an alien force behaves according to Bernoulli's equation, doesn't it? The Venturi effect would produce a lower inward pressure along the barrel, tending to reduce the diameter of the barrel -- greater inward pressure with greater flow velocity -- not the case with the rapidly and violently expanding powder gas, which Dr Cranz found to be expanding the barrel behind the accelerating bullet.

So I wonder whether any conjecture about laminar flow is appropriate to the expansion of the propellant gas. Can the rapidly and violently expanding powder gas flow laminarly? Or is its "flow" different in this respect also?

As if... <img src="/ubbthreads/images/graemlins/grin.gif" alt="" /> <img src="/ubbthreads/images/graemlins/grin.gif" alt="" /> <img src="/ubbthreads/images/graemlins/grin.gif" alt="" />

Because we are, in general, discussing super-sonic speeds here the shape of the base is not relavent to the bullets' air resistance. The air cannot refill the area behind the bullet rapidly enough to affect the bullet.

The supersonic nature of expanding gas is very different from ambient pressure air and acts under very different rules. Of course swapping the speeds of the two will make them change their flow characteristics. In general, gases cannot flow faster than the speed of sound without adding a tremendous amount of energy.

Subsonic bullets do benefit in relation to drag by a tapered base. There may be other factors which work against them there though.

As an example, my father saw how a soda can with both ends cut out could be thrown clear to the next county with amazing ease and attempted to duplicate it with a 12 gauge slug shaped like a tube... that was where he learned a subsonic projectile responds differently from a supersonic projectile.
art

Ken,



I agree with your comments. In general, it takes little air velocity to go turbulent. I can't imagine any circumstance where a bullet in flight wouldn't be affected by turbulent forces.....not laminar (useful velocities).



Wow, the barrel expands behind the accelerating bullet! I've never considered that. Reminds me of a question I've wanted to ask. I've read several posts on this board about getting great groups immediately after a new barrel is installed. My limited experience has been that a new barrel "settles in" and starts grouping after several rounds have been fired........maybe 70 rounds (give or take) . What has been your experience?



HogWild

And RickBin,

How did the subject of this post change from "Why aren't bullets pointed at both ends?" to "Re: Re laminar flow"???? <img src="/ubbthreads/images/graemlins/confused.gif" alt="" />

HogWild

You just change the subject. In the "Subject"' box....

Scott

http://gscustom.co.za/

I�m outta my league here with folks such as Ken and Sitka Deer, but I have a few thoughts/questions on the topic:

1. A bullet pointed at both ends would be long for it�s weight � at what point would the SD and BC be compromised?

2. Comparing two bullets of equal weight, a bullet pointed at both ends will have less bearing area for the rifling to act on and offer less of a gas seal. Less bearing area could also allow the bullet to cant in the bore.

3. I question whether laminar flow can exist in a rifled barrel.

� The shape that's most stable in flight is the sphere -- but it's not aerodynamic enough to be a good bullet. It's also the shortest for its weight -- one factor in its stability.

� The shape that's most accurate is the cylinder -- but it's not aerodynamic enough to be a good high-velocity rifle bullet.

� The cylinder with an added ogive makes the accurate basic bullet aerodynamic.

� The main advantage of the "boat" tail at supersonic velocities is the eye appeal that undergirds customers' confidence.

� The shape of the ogive is important for aerodynamic flight. The shape of the base is important for clean, simultaneous exit from the muzzle.

By God!! We are finally getting to the crux of the matter here!!!
My question is, as a consumer, how can I tell when my confidence is being undergirded, and is that a good thing or a bad thing? Does undergirded come in degrees of undergirded, like insanity, or is it more like pregnancy, one is either undergirded or not undergirded?? Is it possible to be overgirded??
Anxiously waiting a clarification.

TIC

Royce

The best confidence is that which has been eroded, subsequently researched, and ummm...re-undergirded. Anything else is blind faith.

I prefer bullets with a high BC because, well, that seems better than a low BC. That said, I don't get to cranked up over a BC of 0.3 as compared to a BC of 0.4.

Ken ... do the math .... .154 squared times pi equals .0745" squared (inches squared). Gee, just as I typed!



I do however agree with your opinion of laminar flow. But am curious on how you can say gas cutting would be more prevalent if the contact area of bullet and barrel remain the same between both types of bullets. To say they are different are an apple and oranges type of thing.

Sorry, 'gadro -- I'm more persnickety with my terminology. When I learned my wee modicum of math back around the Pleistocene, a number squared equaled so many (different number) square inches. So I misread what you typed, I guess. Is your use of "squared" a term of the new math?

Understood ... just when I use units in chemistry I'm used to using them as written (ie. inches followed by a superscript 2). But when talking construction and such I usually use square feet and not feet squared.

Such when reporting data where I may have 5,000 events / cm^2 ... is said normally "five-thousand events per centimeter squared" also is often refered to as "five-thousand events inverse centimeters squared". Depends on with whom your talking too, and the environment, on how each gets used.

Quote:
___________________________________________________

I've read several posts on this board about getting great groups immediately after a new barrel is installed. My limited experience has been that a new barrel "settles in" and starts grouping after several rounds have been fired........maybe 70 rounds (give or take) . What has been your experience?
___________________________________________________

Any thoughts? Anyone?

HogWild

HogWild,
I think after about 70 rounds, you've probably cleaned the rifle enough times for bullets to "wear in" on a clean bore surface, smoothing out the tooling marks in the new bore. You can accomplish the same thing by "breaking in" a new bore, cleaning completely after each shot. Your first few rounds will have a lot more color on the patches, but as you progress, the first patches from a few more rounds will be a lot cleaner.

Regarding the pointed bullets at both ends, if a flat portion is better for the gas to push against in a boat tail, would a concave (hollow point) in the rear bullet be any better? Or is this too much gas on an already gassy subject?

WinMike: A "rebated boattail" seems to be something like that at Swage.com. Although, just because they cite the advantages of the design as precluding laminar flow doesn't mean laminar flow actually exists.



The phenomenon does seem to exist, however. The Corbin website cites the Lapua rebated boattail, originally designed to combat laminar flow.



Jaywalker