SparticleBrane:https://www.deepseasupply.com/index.php?product=54
Perfect, exactly what you're looking for. Essentially a sawed-off steak knife with a blunt tip so that it won't poke your exposure suit. VERY secure sheath. Sharp knife, solid end if you want to bang it on something.
I have one of those.
The OP asked for something medium sized (that knife is pretty small) and also with a metal butt.
I'll try banging my tank with it this weekend but I don't think it would massively effective, especially as it's so small you would have to grip the cutting edge to bang using a fist, rather than two fingers & a thumb to hold it.