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dummyracer36 · Senior Member

this may have been covered already, but the search funcyion confuses me more than it helps.

it is my understanding that the classic diablo shaped pellet stays stable below the speed of sound due to air friction, and becomes very erratic and unstable near or above the speed of sound.

now some of the rifles out there are more than capable of sending pellets down range well above the sonic barrier. would a more "bullet" shaped or "boat tail" pellet be stable at these speeds?

i'm asking because i am considering casting my own pellets from some tooling i am ordering from Corbin Industries. they offer molds for classic diablo and for boat tails (along with the swaging epuipment for final sizing).
but if the boat tails simply wont work, i dont want to bother wasting the money on the molds.

any opinions or actual data would be greatly appreciated

Amigo · Veteran Member

Racer:

What type of air rifle were thinking of using the boat tail "bullets" in?
Success could be very iffy in a springer while there might be a little better chance with a pneumatic.

Amigo

dummyracer36 · Senior Member

Amigo, would be mainly for the PCPs, but if i have em on hand... dont see a reason to not test them in some of the springers.

Amigo · Veteran Member

A PCP would probably be the most likely to propel a bullet. With more available propellant volume, the greater surface area friction drag of the rifling on the bullet periphery might be over come.

Good luck with your experiment.

Amigo

Slavia · Moderator

I would certainly try it, but I think you would need a higher twist rate than airgun barrels provide to fully stabilize that elongated projectile. See "Greenhill Formula":

http://en.wikipedia.org/wiki/Rifling

thepaladin · Senior Member

I read somewhere about a new line of commerically available "bullet style" pellets for the higher poered guns both PCP and springer. The answer to the diablo shape is of course to use haevier pellets and keep the spped down slightly below sound speed. Don't really know how the bullets will work. If you try the boat tails I'd be curious to know how they work.

Jaxom · Senior Member

DR,

I would be very interested in your experiment as well. You mentioned about having molds machined. I was thinking along the lines of cast ones myself. If I may ask, what made you think about doing this in the first place?

Two things did it for me. The first was someone I spoke to on another forum where backyard casting foundries is the forums subject. One of the members there gets used wheel weights from tire shops. He takes them home, melts them down and then casts shot for several shot gun clubs near him.

The next thing that put this in the back of my mind was seeing the sinker molds in my spring Bass Pro cataloge. I started to wonder, what if....

Since it seems by your post you're concidering a different shaped pellet, have you also concidered which lead alloy you will be using? This will have an effect on the effency as well.

kanyon · Silver Status Member

The hour glass shaped pellet do stay more stable
at sub sonic speeds but as the speed drops the stabillity
will incresse...
Ok..from my flying days...Lets see how my memmorys
holding up....
It has do to with 2 facters...head shape & center of G
a pellet needs to produce drag at the rear to keep it stable
hence the shape....but once the flight speed gets to fast the air
flow over the nose of the pellet will produce neg pressure
zone at the sides of the pellet..with reduces the drag at the rear....
Now you have a pellet with pressure on the noise and no drag at the rear
so the front wants to become the rear... so it turns slightly then the rear regains drag so it straightens up...and this happens over ond over till
the motion over rides the drag / pressure then the pellets starts to tumble.

this is even more noteiceble in wadcutters with the flat noise as the low pressure zones are produced at even lower speeds.

I think this is why Einjins can be fired super sonic with very little efect on them....all the ribs around them produceing a lot of drag along the sides.

Now if you are thinking.....why can a bullet fly at above super sonic speeds?.....becuse it is traveling in a low pressure void....
at super sonic speeds a pressure wave is travelling in front of the bullet & there is now very little pressure on the noise...when these pressure wave are produced is where the sonic boom comes from.

Amigo · Veteran Member

Kanyon, you NZ guys know your aerodynamics and your physics Exclamation

Maybe that why NOBODY messes with NZ.

BTW, your explanation was well put.

Best regards,

Amigo

Slavia · Moderator

rsterne · Moderator

Here is an interesting diagram I found:

As you can see, the Coefficient of Drag starts increasing at about 75% of Mach (ie just over 800 fps, as Slavia says).... It keeps increasing until about Mach 1.2 (~1300 fps).... This means that the meager ballistics coefficients of our pellets (about 0.010 to 0.035) gets even worse in the transonic region.... Shocked

A .22 LR bullet has a ballistics coefficient of 0.145 (ie 5 to 15 times as good as a pellet).... and they still fall subsonic before they get to 100 yards.... Rolling Eyes

Bob

Jaxom · Senior Member

Thanks rsterne for proving me wrong!!! I argued with the nuns that tried to teach me algebra over and over, that I will NEVER need to know this stuff in the real world... OY!

broommaster2000 · Moderator

If you look at those wads of led that they put in .50 air rifles... Mr. Green

Those aren't Diabolo shaped.

kanyon · Silver Status Member