An experiment I would like to see

[jps2486]

Hot rodders know that a very smooth surface inside the intake maifold of engines is less efficient than a rough surface, so they grit or bead blast the interiors before using. I wonder how much different pellet velocity would be between as manufacured pellets and those with bead blasted heads? Just a random thought.

 

[dgeesaman]

JSB Hades are basically a roughed up diabolo and they have significantly lower ballistic coefficient.

Reynolds number describes the intensity of turbulent energy in a fluid flow. There are a few narrow ranges of Reynolds number where roughness reduces drag. Golf balls size and velocity happen to fit it perfectly. There aren’t many others in practice - smoothness is usually better.

If you want to try it maybe press and roll the head of a pellet into very coarse sandpaper?

 

[Long_Gun_Dallas]

On porting of my FX Crown I polished the port just to see. I had already reworked the ports on it prior. Of course there was a negative effect. Can't recall exactly how much, but I want to say ~15fps vs unpolished. 930fps was the best it was doing on it's tune with 18.13gr in 380mm .22 barrel. MK1 Crown. 125 or 135 bar. 130 picked up more power and was less efficient, I like running 135. Whatever that pressure is in actuality, I have no idea, just what the gauge says.

Also, wanted to say that in my experience, the .22 hades seem to noticeably lose velocity quicker. I could just be imagining things, but would be interesting to run them across a lab radar against the 16gr AA pellets.

 

[Gerry52]

It only makes sense to me that the Hades dome design will be less aerodynamically efficient than a smooth domed head. Hades might lose a little more speed but are still devastating when they hit.

 

[Ballisticboy]

If you look at the surface of a pellet at high magnification, you will see that it is anything but smooth on just about all pellets. Just look at the pictures in the thread comparing the surfaces of Zan .3 calibre pellets and slugs, and they look relatively good to me. The air will react to the smallest surface imperfections, as anyone who has worked with wind tunnels will know.

As for Reynolds numbers, you are going to be far more concerned with Mach number effects at pellet speeds, particularly at the pellet speeds used in the US. At slower speeds, the main drag component comes from the back of the pellet, the nose provides relatively little of the total pellet drag.

 

[dizzums]

i think when dealing with air/charge volume, the idea is a rough surface allows it to 'stack' better and more densely in the runner instead of just slipping to the shortest side ..id say a mirror polished dome would slip through the air better than a multi-faceted front surface .. but im guessing based on intuitive logic ..i could be dead wrong lol

 

[Ballisticboy]

This is a breakdown of the drag coefficients for the nose and the flare of the .177 JSB Heavy pellet, which has a relatively blunt nose with a higher nose drag than the nose of say the JSB 8.4 grain pellet. There are a few additional minor components in the total drag figure.

Speed Ft/Sec​	Nose drag​	Flare drag​	Base drag​	Total Drag​
334.95​	0.045​	0.003​	0.225​	0.332​
558.25​	0.055​	0.004​	0.23​	0.346​
669.9​	0.065​	0.004​	0.232​	0.357​
725.725​	0.075​	0.005​	0.233​	0.368​
781.55​	0.087​	0.006​	0.235​	0.382​
837.375​	0.113​	0.007​	0.2365​	0.410​
893.2​	0.142​	0.008​	0.238​	0.441​
1004.85​	0.217​	0.011​	0.241​	0.523​
1116.5​	0.38​	0.013​	0.281​	0.739​

You may be firing your pellet at 900+ ft/sec, but it soon slows down to much lower speeds where you can see you will be much better off concentrating on reducing the drag of the back end of the pellet, not the front.

The numbers were calculated, but then calibrated against firing data of measured pellet speeds. Most unusually, the measured and calculated speed loss figures were extremely close to each other, so no adjustment was needed.

If you wanted to use the drag law in a trajectory model which uses BC, you will have to change the BC to match the sectional density

 

[Airgun-hobbyist]

The negative effect from the air/gas mixture adhering to the smooth surfaces in an intake manifold., and why we keep plenum runners rough when porting.... I can see a rough peened dome keeping the air from sticking to the dome of the pellet may have some long range benefits in the wind drift department, and beneficial to accuracy. I don't see it changing BC too much though.

 

[kevtan]

That is just so cool!

 

[Scotchmo]

A rough surface can promote turbulent flow. That might be good for down stream of the carburetor in an IC engine. Keeps the fuel atomized. I can’t imagine any benefit for a direct injection IC engine.

For lowest drag in an airgun projectile I’d say smoother=better

 

[Ballisticboy]

For lowest drag in an airgun projectile I’d say smoother=better

Unfortunately not true on projectiles as smooth flow is less able to follow the contours of the projectiles and thus tends to break away sooner creating more drag, particularly base drag which is the biggest component of the total drag.

Smooth flow will also often transition at some point to turbulent flow, depending on the Reynolds number and projectile shape. The transition point has a nasty habit of moving around, which can give rise to stability problems. A nice area of rough surface can fix the transition point and avoid stability problems. Many aircraft have used such a surface to help stability and controllability.

 

[Ballisticboy]

I can see a rough peened dome keeping the air from sticking to the dome of the pellet may have some long range benefits in the wind drift department, and beneficial to accuracy. I don't see it changing BC too much though.

If it doesn't change the BC, it will not change the wind drift either.

Also remember, a turbulent flow will follow the projectile contours far better than a smooth laminar flow.

 

[357cal]

As for your curiosity about pellet velocity, the difference would likely be minimal. Bead blasting primarily affects surface finish, not internal ballistics. The velocity of manufactured pellets versus bead-blasted ones would likely remain similar, as the blasting process doesn’t significantly alter the mass or shape of the pellet heads. However, for precise data, testing would be necessary.

 

[Scotchmo]

Unfortunately not true on projectiles as smooth flow is less able to follow the contours of the projectiles and thus tends to break away sooner creating more drag, particularly base drag which is the biggest component of the total drag.

Smooth flow will also often transition at some point to turbulent flow, depending on the Reynolds number and projectile shape. The transition point has a nasty habit of moving around, which can give rise to stability problems. A nice area of rough surface can fix the transition point and avoid stability problems. Many aircraft have used such a surface to help stability and controllability.

Most all projectiles I use have a relatively smooth surface. How rough does the surface need to be in order to see benefits?

 

[357cal]

Most all projectiles I use have a relatively smooth surface. How rough does the surface need to be in order to see benefits?
View attachment 470542

While I don’t have specific data on bead-blasted pellet velocities, I can offer some insights based on principles of internal ballistics. Bead blasting mainly affects the surface texture, not the mass or shape of the pellet head. Here’s why the velocity difference would likely be minimal:

1. Mass Consistency: Bead blasting doesn’t alter the weight of the pellet. Since velocity depends on mass (among other factors), consistent mass minimizes velocity variation.
2. Shape Preservation: The blasting process doesn’t significantly change the pellet’s shape. The head remains aerodynamic, affecting velocity less.
3. Internal Ballistics: Pellet velocity primarily depends on air pressure, barrel length, and bore diameter. Surface finish plays a minor role.

For precise data, empirical testing with various pellet types (manufactured vs. bead-blasted) would be necessary. Keep in mind that individual pellet designs and airgun setups can vary, so results may differ. We are talking about ammo and not barrel right?

 

[Scotchmo]

Wouldn’t a properly sized, smooth boat tail maintain attachment at the rear better than any other “rough” shape?

 

[357cal]

Wouldn’t a properly sized, smooth boat tail maintain attachment at the rear better than any other “rough” shape?

I am in firm agreement, as this has been demonstrated repeatedly. Boat-tail slugs have a tapered, streamlined rear end (similar to boat hulls). This shape reduces drag and stabilizes the projectile in flight, allowing it to maintain velocity over longer distances. Rough-surface slugs have irregularities that create more air resistance. They experience greater drag, leading to decreased range compared to smoother projectiles. This is just simple science.

 

[357cal]

Hot rodders know that a very smooth surface inside the intake maifold of engines is less efficient than a rough surface, so they grit or bead blast the interiors before using. I wonder how much different pellet velocity would be between as manufacured pellets and those with bead blasted heads? Just a random thought.

Are talking about the ammo and not the barrel ?

 

[Long_Gun_Dallas]

Most all projectiles I use have a relatively smooth surface. How rough does the surface need to be in order to see benefits?
View attachment 470542

Educated guess would say between 120-200 would be ideal. For larger objects like an intake runner on an engine I'd lean toward 300-420. Honestly, it's a hard concept to put into words. But look at some real world experiments on this, and you'll see the affect for sure. Almost like air or fluid sticks more to a smooth surface than it does to a bit rougher one. Not sure if this relates to a buffer zone created by the pockets or as @Ballisticboy said, that smooth airflow is easier disrupted and more prone to turbulence. Maybe its just just 2 sides of the same coin. Either way, the affect is real and is there.

 

[Ballisticboy]

I am in firm agreement, as this has been demonstrated repeatedly. Boat-tail slugs have a tapered, streamlined rear end (similar to boat hulls). This shape reduces drag and stabilizes the projectile in flight, allowing it to maintain velocity over longer distances. Rough-surface slugs have irregularities that create more air resistance. They experience greater drag, leading to decreased range compared to smoother projectiles. This is just simple science.

Unfortunately, in this case, your simple science is misguiding you. Boat tails produce destabilizing aerodynamic moments about the CG due to giving negative lift when yawed to the air flow, so you require higher twist rates for the same stability factor. Coupled with this is the fact that the boat tail also increases the boundary layer thickness, which will increase Magnus moments, leading to a reduction in dynamic stability.

As for drag, the largest single component of drag for a slug with a reasonably shaped nose is the base drag. It is a well proven fact that turbulent boundary layers at the base of a projectile reduce the base drag compared to that which is produced by a laminar boundary layer. The turbulent boundary layer does this by increasing the mixing of the air in the low pressure area behind the base, increasing base pressure and thus reducing base drag. So, while you may obtain a small reduction in aerodynamic skin friction drag by keeping a laminar boundary layer, you will create a much larger increase in base drag with a laminar boundary layer compared to the turbulent boundary layer. This is basic aerodynamics, as shown by the tests on sphere drag, amongst other things. Slugs and airgun projectile complicate things further by flying at relatively low Reynolds numbers, which can cause the complications above.