Light pellets at very very high velocity?

[subscriber]

MrP,

The images of 3D models below are intended convey what general dimensions a "light" yet practical lead slug might need to have:

I assumed some dimensions that happened to come out at 58.8 grain as a solid; then iterated until I got to the 24.2 grain hollow nose slug that is FX Hybrid-like.  Yes, the nose cavity has a straight wall, so would need a slight taper to prevent sticking on the core pin.  I later added a slight radius to the sharp edge of the nose cavity of the lightest slug, to be more realistic.  That dropped the weight to 24.5 grains.

Obviously I could make the light slugs shorter, but I consider the bearing length of .1875" as short as one wants to go for in-bore stability out of a .25 caliber air rifle.

The little green blob showing the XYZ axis indicates the center of gravity - in case that was not obvious.

The green outline is of the 58.8 grain solid slug superimposed with the 25.2 grain hollow nose one inside it; matched at the slug base.

[subscriber]

The video below explains the value of a heavy material near the OD of a light, yet high BC, spin stabilized projectile.  The purpose of the "heavy jacket" is to maximize the projectile's rotational inertia about its long axis; so that spin speed does not decay significantly for such a light projectile.

In the case of the video, it is an aluminum "core" spitzer bullet with a copper "jacket", driven at over Mach 2. 

In your case, I suggest the slug "core" be comprised of air, with a lead "jacket", rather than copper. A 24.5 grain slug at 1500 FPS yields 122 FPE.  Slightly de-rated from your 130 FPE, achieved with a heavier slug, due to lower efficiency with the lighter one:

[subscriber]

MrP,

Amazon (Rotometals) sells pure lead sheet 1/32" (0.0313") thick: www.amazon.com/gp/product/B001QV3EUA. 



If you are feeling frisky, you might swage discs punched out from that sheet into a 23.45 grain hollow slugs, as modelled below.  One could use hydroforming to partially close the nose, without trapping any tooling in there... 

A 23.45 grain slug travelling at 1550 FPE would yield 125 FPE. 

As you mentioned 16 grains in your OP, I tried to model that.  A slug with a 0.0313" thick lead jacket with an overall length of .25", and a bearing length of 0.125", with a nose radius of .25", results in a weight of 16.52 grains.  At least the center of gravity of this squat little slug is still behind the front end of the full caliber cylindrical section (because of the solid base and rounded nose). 

At 1800 FPS and 119 FPE (unlikely that fast, due to poor efficiency), this slug should be devastating at 10 yards against fragile targets.  But, it probably won't group well at 50 yards.  If you made dies to make the 23.45 grain version, you should be able to make the 16.52 grain slugs too...

[subscriber]

MrP,

I will shut up after this post...

See this thread about attempts to shoot slugs supersonic from a PCP:  https://www.gatewaytoairguns.org/GTA/index.php?topic=184967.msg156116230#msg156116230

Then brace yourself for stern advice not to waste your time shooting airguns slugs over 1050 FPS.  Turned into a question; what is the heaviest .25 slug you can shoot at 1050 FPS?

[rsterne]

Already given on page 1....   

Making a hollow lead slug is an excellent way, requiring more than one pass through a swaging press, of course.... The diameter of the resulting Meplat will determine the balance between better ballistics (50% or less) or ease of manufacture and hitting power at close range (75% or more).... Above Mach 1 you would want to stay with a VERY small Meplat (25% or less).... which I would think would be very hard to make with a large cavity....

Bob

[Mr.P]

Creative and detailed info!  I am not skilled in manufacturing so for the time being the range of my possible tinkering is likely just to change the metal I cast with.  Sounds like a dead end...

[ballisticboy]

The large meplat slug shape is not one you should be using at speeds above Mach 1, the drag will be huge.

[Mr.P]

"what is the heaviest .25 slug you can shoot at 1050 FPS?"

If that was directed at me I need to check. 50 grains for sure. I can get 60 grain up close to 1000 but not quite I think.   I have 43.5, 48, 50, 58, 60 grain options. I should see how fast the 58 grains go (HP versions of 60 grain slugs)

[subscriber]

Above Mach 1 you would want to stay with a VERY small Meplat (25% or less).... which I would think would be very hard to make with a large cavity....

Bob, I would fill the open lead cup with water, then use water pressure to create the counterforce required by the nose forming die.  After forming, the water is drained out and given time to evaporate. 

It just occurred to me that the water could be left inside, and the nose of the slug crimped shut - then sealed with lacquer.  The weight of the water would have to be included in the equation, but if it could be managed consistently, that  should make for some pretty strongly expanding projectiles that could have a relatively sharp nose for super sonic travel    .

Here the principle of hydroforming is demonstrated, to expand the shoulder area of a thin walled cartridge case:

[rsterne]

Good idea, I look forward to seeing successful production of airgun slugs using that technique.... Launched at over 2000 fps using Helium they could work well.... 

Bob

[darkcharisma]

I wonder how much Helium is worth now per KG... probably unreasonable