A overdue follow up for the question Metallic Hydrogen Propellant.

One of the points that was brought up on the post was that since the gas propelling the bullet would be atomic hydrogen, the effective exhaust velocity would be much higher than conventional propellants used today. this could, in theory, lead to an overall decrease in barrel length for an equal amount amount of performance in rifles today.

The question I have is, by how much would the barrel decrease, if at all?

Please note, this question is very much conncested to the linked one, along with the assumptions and results as well.

  • $\begingroup$ the metastable (liquid) metallic hydrogen would, upon a (thermal?) stimulus, become gaseous again, but what do we know about this process? It is pure handwaving to say that mh is stable at normal pressure&temperature to begin with, so the behaviour while spontaneously changing back is up for grabs, right? So we might as well ask about a projectile being propelled by 400GPa of atomic hydrogen. So we handwave the chamber. but if we do that, we handwave the reason for 'barrel-length=accuracy' as well, as we can posit a perfect fit, resulting in perfect accuracy from a bullet-length barrel $\endgroup$
    – bukwyrm
    Commented Jan 7, 2022 at 13:22

1 Answer 1


Barrels serve two purposes.

One, as you note, is conversion of propellant energy into bullet energy. Faster propellants mean you need less barrel for the same final velocity.

However, there is also an accuracy issue. The shorter the barrel the less accurate the weapon. That is independent of propellant velocity.

Thus, depending on the purpose of your gun you might not shorten the barrel at all.

  • $\begingroup$ Interesting point about accuracy, but what about the muzzle velocity of the bullet? It would be obviously higher, but by how much is something I'm wondering about. $\endgroup$
    – Seraphim
    Commented Jan 7, 2022 at 21:05
  • $\begingroup$ @Seraphim I don't know the answer to that part of it. $\endgroup$ Commented Jan 8, 2022 at 2:30

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