The cold will bother you.
OK. Some math! I have a bad habit of letting those decimal points slide around so anyone interested in checking the math - please do.
Metallic hydrogen: 0.7 g/cc
Mole of gas at STP = 22.4 l
Boiling point of hydrogen = -252 C
1 mole gas at -252C and 1 atm = 1735 cc
1 atm at STP = 0.191325 Mpa
1 mole gas at -252C and 1 cc = 175.8 Mpa
0.7 mole gas at -252C and 1 cc = 123 Mpa
1 Mpa = 145 psa. 123 Mpa = 17835 psi
So 2 cc of metallic hydrogen would give you 35670 psi
I had to convert to psi to use this fine chart of various guns and muzzle velocities.
http://closefocusresearch.com/calculating-barrel-pressure-and-projectile-velocity-gun-systems
I use the chart because muzzle velocity will depend on barrel length and projectile weight. The barrel pressures here are those generated by a standard propellant load for that gun type and I swapped in the pressure generated by the metallic hydrogen sublimation.
35670 psi is close to the 33601 psi for the 357 magnum with muzzle velociity of 1250 ft/sec.
I found that the 357 magnum has a bullet case capacity of 1.7 cc. We had to use 2 cc metallic hydrogen to get the same muzzle velocity. So for guns you might be better off using black powder.
The assumption I made that made the metallic hydrogen weak is the temperature. When you let a liquid (or solid) expand into a gas you have an increase in enthalpy and so a decrease in temperature. That is why a can of liquid air gets so cold when you discharge it. The more the expansion, the more the pressure drops. Given the phenomenal increase in volume with the phase change from solid to gas hydrogen I assumed the gas would be at the lowest temp that would allow the phase change to happen which is the boiling point of hydrogen. Low temperature gases occupy less space and so produce less pressure. Because it is so stinking cold it is not as energetic as the black powder at 0C.
More math, because Starfish says H+ gas will turn into H2 and heat up. And expand.
Heat of formation of H2 from H + H = -104 kJ / mole
Specific heat of H2 : 28.8 J / mole / degree K
-252 C = 21 K
Let us start with just 1 mole of H2 gas.
104 kJ = 104000 J / 28.8 = 361 K
The gas was 21 K to start with so that gives us H2 at 382 K or 108C. Quite hot indeed.
Back here: https://www.calculatorsoup.com/calculators/physics/ideal-gas-law.php
0.7 moles gas in 1 cc at 108 C = 21893 atmospheres. That = 321738 psi.
So an order of magnitude higher than what the conventional firearms propellant use. That sounds more like what I have heard about metallic hydrogen - thanks Starfish.
Seraphim, muzzle velocity depends on things like muzzle size, ammo weight etc. If you have firearms like those in the table you could use just 0.1 cc (0.1 ml) of metallic hydrogen to divide pressure generated by 10 and then wind up with muzzle velocities on par with the firearms here.
