since they became Puella Magi, their bodies are just a shell and nothing more than just a tool to fight the witches. Plus, their bodies need to be within a radius of 100 m from their Soul Gems (which are exactly what their name says: PM’s souls are cristallized as SG when they accept the contract with Kyūbey) to stay alive.

Thus, Homura knew that she wouldn’t die if she’d shoot herself with her gun, and this is why she did it. And she also knew that her SG would heal her wounds after a while

Uhmmm... I do vaguely remember about the soul gem and stuff, but I believe at some point in the anime Kyubey mentioned that by separating the soul from the body it granted "Extra Durability" but not semi-immortality, I mean.... they're still using their brain to function, even if they don't die from being shot in the head and even in the case that "eventually" they heal back, they'd be still totally incapacitated for an extended period of time, I wonder how Homura's plan would have worked like that