Couple weeks ago I posted about this. But I was being hasty. As my numbers were only simulated.

Week ago I posted that I exceeded the muzzle energy of 380ACP with unmodified 8mmK blank.

426.1m/s or 295 joules.

I shaved the 10mm of the handle. Making the total lenght 17.3mm.

Sometimes the steel cases stick so hard to the chamber that they need to be violently hammered out. Hammer is not necessary but something with heft is needed.

Gemini estimated the chamber pressure to be 3800 - 4500 bars with the unomodified steel case 8mmK blank.

I had one brass cased 8mmK blank laying around as I had measured it's internal volume. (It's higher than in the steel cased blank).

I used 208mg of double base propellant from 25 caliber nailgun blanks. The strongest short Black loads have 200mg. That seemed like a good amount for testing.

I am used to making 400mg charges but that's in my 9mm bore zip guns.

I know that the pressure inside 6.35mm bore (mine is sligthly under 6.3mm). Is very high.

My intention wasn't try to exceed the 9mm parabellum power. I just wanted to test if the brass shells eject more easily after high pressure firing.

And they do indeed eject easier. Some light taps with the handle portion of the zip gun was enough. No need for violent hammering.

The measured velocity and power exceeded my expectations.

581.4m/s or 550 joules out of 70mm long (short) barrel.

Exceeding 9mm parabellum energy by wide marging.

I asked Gemini what was the chamber pressure. It calculated from the average pressure with multiplier of 2-3 for blank powder. Resulting in chamber pressure of ~7000 - 10 000 bars. Absolutely nuts.

The nailgun blank powder probably sits in the higher end of the estimate. In the nailgun there's only ~1.5cm long distance to accelerate the bolt before the gas is vented. So it's extremely aggressive propellant. I also compressed the charge. (That's how they are in the most powerful blank loads).

My zipgun has calculated barrel strenght (rupture pressure) of little over 10 000 bars. Very little safety margin.

Rupture pressure for very short peak impulse is about 1.5 times higher than the calculated burst static pressure.

But the bearing that I have used to reinforce the barrel doesn't like expanding. So better not push it further.

I am not a fan of the bullet keyholing and blank powders and rifling do not mix well.

But 7mm tungsten carbide ball weighs 3.2 grams. Almost exactly the same amount as 25ACP bullet.

The larger bore gives bit room to increase charge. But at the same time the steel cases have smaller internal volume.

I want to hit target velocity of 600m/s.

That should be within safe limits.

It has been studied that about 600m/s is optimal impact velocity for handgun bullets. The study wasn't made public so not sure about the results. But I suspect it's a sweet spot kind of deal. Higher velocity is harder to achieve. Not worth the extra lethality.

7mm tungsten ball has similar frontal area than the 7.5FK F9 bullet which is the most lethal handgun bullet. At 3.2 grams it's lighter and has less muzzle energy but should be devastating and penetrate deep enough in a human target.

7mm tungsten ball flying at 600m/s also pierces level IIIA body armour with ease.

So have I now perfected the zipgun/pengun?

Tungsten carbide balls will fix the keyholing issue and provide armour piercing capability.

I could increase the energy further with 32 caliber 4.6 gram bullets. But those will keyhole. So the wolfram balls are superior.

I could probably still shorten this. But the 90mm long handle is already short. Shortening it will ruin ergonomics. The barrel could probably be shortened without much loss of power as the blank powder burns so fast. But the design is already short enough. It's very conceivable.

What do you think?