I'm currently designing the armory for my far-future hard-scifi setting. Technology is quite advanced and for the purpose of this question assume that liquid metallic hydrogen (LMH from now on) is actually metastable, and diamonoid material tanks capable of holding it are a thing. The technology was developed for spacecraft propulsion as LMH is the best chemical rocket fuel (actually uncertain about how to call it, since no chemical reaction is used, just a phase change; any clarification would be appreciated). All technological problems concerning the LMH part of the question are to be considered already solved.
The actual design would work as follows.
a diamonoid material shell containing the LMH is inserted into the barrel; it is near the edge of the pressure it can hold
it is propelled down the barrel, currently I'm thinking that pressurised hydrogen gas would serve me best
a laser in the back of the barrel is fired to heat the shell so the pressure inside it increases and the diamonoid material is weakend; the bullet is now armed and while it will still survive travelling though air, a direct impact will break it
the impact on the target breaks the bullet, the rupturing of a high pressure pressure vessel, the highly energetic phase change to H2 gas (I read once that the rocket exhaust of LMH would be at 7000K) and a number of chemical reactions initiated by the hot, reactive hydrogen will wreak havoc on the target; I'm uncertain about what exactly would happen on impact, if someone would explain the process which is likely to occur in detail, it would be nice
I'm aware that this isn't really a real plasma weapon in the spirit of star wars, but a ludicrous grande launcher. Yet considering the nature of the explosion some plasma might be involved. Whatever the case, the marketing department decided to call it a plasma weapon, so it is a plasma weapon. Furthermore I'm not trying to make a practical infanterie weapon here. I'm aware that enemy snipers would make it a sport to shoot your ammo supply. However this weapon will be great if my protagonist has to show of his manliness on a monster infested jungle-world. Or if you are not in the mood to dispose of the body and don't mind major scorch marks. Additionally the gun, or lets be real here, the portable light atelery piece, should have a feature prohibiting it to be shot at a point less than 20m from the user.
Is the design plausable? Could I improve it somehow? Will it be as effective as I think?