Bullet Smoothness

[M-61]

Keith,
A-4 Skyhawk. Not a wire but vortex inducing "device".

Vortex generator
Fairly common for many decades

 

[HappyHellfire]

When I started turning bullets on my lathe they would every once in a while produce a rough surface like they were 3D printed. I have been just throwing them away. Now I think I want to intentionally make some and test them.

 

[Kracken McGraw]

I figured the dimples on the golf balls had something to do with creating surface for the club to hit. What’s a good group on those at 400?

 

[butchlambert]

How do you handle weight to length of your bullets? I see your bullets are very light in weight.

 

[HappyHellfire]

How do you handle weight to length of your bullets? I see your bullets are very light in weight.

I'm starting with solid copper because it is much cheaper to break into the market. I hope to eventually expand to lead core and make them heavier.

The weight to length models for stability are simplification of a method called linear projectile theory that I can pretty accurately calculate stability. In general though, you put the pointiest nose and aerospike and then keep adding bearing length to get the desired weight.

 

[butchlambert]

I'm starting with solid copper because it is much cheaper to break into the market. I hope to eventually expand to lead core and make them heavier.

The weight to length models for stability are simplification of a method called linear projectile theory that I can pretty accurately calculate stability. In general though, you put the pointiest nose and aerospike and then keep adding bearing length to get the desired weight.

I understand that, but you will be making bullets at the desired weight that you will have to pull the bolt to load and extract a loaded cartridge.

 

[butchlambert]

Check this link. https://www.warner-tool.com/flat-line-bullets/ Alan is as good as it gets.

 

[Webster]

A “rough” surface texture aids creating a boundary layer of air around the bullet or any object for that matter and improves the aerodynamic properties. Slick polished surfaces are “stickier” rough and create more drag in fluid dynamics. Examples of this are things like boat hulls, golf balls and cylinder head ports. No polished surfaces create less drag once objects are set into motion.

That's why fighter jets have dimples like a gulf ball.

 

[divingin]

It may sound counterintuitive but very smooth might not be the best option. I remember seeing something about racing yachts being faster with a slightly rougher surface like shark skin. Air may behave differently to water of course.

Look at golf ball design as well. The dimples make for less drag as the ball flies.

Sorry, Webster; didn't see the second page of replies.

 

[Doom]

Golf balls use dimples to create a turbulent boundary layer which delays the separation of the flow versus a a laminar boundary layer. In this case the rough surface does improve the total drag at the expense of the surface friction drag.

 

[divingin]

Doom: In shooting, as in golf, the goal would be to decrease the overall drag, would it not? As a shooter (or a golfer), you care about the overall system performance, rather than breaking everything down to components of the overall drag (unless you're trying to improve a single aspect of that; but most shooters aren't bullet designers, so don't care.)

 

[Doom]

Doom: In shooting, as in golf, the goal would be to decrease the overall drag, would it not? As a shooter (or a golfer), you care about the overall system performance, rather than breaking everything down to components of the overall drag (unless you're trying to improve a single aspect of that; but most shooters aren't bullet designers, so don't care.)

You are correct. However it's a cost versus return balance. If the total percentage of fictional drag is 10% (guess) then making a 1% improvement in that drag is only 0.1% improvement in total drag. From a manufacturing standpoint if that were to add an additional process it may well not be worth the cost.

To the point of optimizing the entire process, the first thing you have to do is determine what measurable goal you are trying to arrive at. You actually have to break it down to the individual components such as the various drag sources or brass prep/reloading processes or whatever. In doing so you can determine which processes have the greatest effect and where to put time, effort and money to get the greatest return. It is also necessary to determine if the expected gain can actually be measured to be statistically significant.

 

[HappyHellfire]

You are correct. However it's a cost versus return balance. If the total percentage of fictional drag is 10% (guess) then making a 1% improvement in that drag is only 0.1% improvement in total drag. From a manufacturing standpoint if that were to add an additional process it may well not be worth the cost.

To the point of optimizing the entire process, the first thing you have to do is determine what measurable goal you are trying to arrive at. You actually have to break it down to the individual components such as the various drag sources or brass prep/reloading processes or whatever. In doing so you can determine which processes have the greatest effect and where to put time, effort and money to get the greatest return. It is also necessary to determine if the expected gain can actually be measured to be statistically significant.

Currently, the best estimate for long pointy bullets is 5% friction drag, 45% wave drag, and 50% base drag. That is one of the reasons I kept working on aerospike bases over the years. It has a high potential to reduce overall drag.

 

[HappyHellfire]

I've noticed that about a week after I make my bullets they start to go dull with surface oxidation. It doesn't really effect the surface roughness but should I coat them to keep them shiny?

 

[butchlambert]

I've noticed that about a week after I make my bullets they start to go dull with surface oxidation. It doesn't really effect the surface roughness but should I coat them to keep them shiny?

NO!!

 

[10XSHOOTER]

He maybe talking about the STATIC leads on the trailing edge of a wing.?
They don't help anything except to discharge Static Electricity.

 

[Newhouse]

@HappyHellfire , check the surface roughness of your baseline factory bullets and make yours the same. Measure them with surface roughness meters which maybe available at your university. I’m guessing between 8 to 16 Ra.

 

[HappyHellfire]

@HappyHellfire , check the surface roughness of your baseline factory bullets and make yours the same. Measure them with surface roughness meters which maybe available at your university. I’m guessing between 8 to 16 Ra.

I can do that. What I am wondering is if they "should" be smooth? The pressing/swaging operation used to make most bullets leaves the smooth surface. Would it be better to have something else?

 

[HappyHellfire]

He maybe talking about the STATIC leads on the trailing edge of a wing.?
They don't help anything except to discharge Static Electricity.

I looked but couldn't find any pictures of this online. I know when we run wind tunnel tests on airfoils we often use the "trip wire" on the leading edge to initiate a fully turbulent boundary layer.

 

[Dave M.]

@HappyHellfire , check the surface roughness of your baseline factory bullets and make yours the same. Measure them with surface roughness meters which maybe available at your university. I’m guessing between 8 to 16 Ra.

Pretty sure none of us possess this equipment at home and my work frowns on me using the Met Lab.