Leonidas was a Psyker, and what he just did, was because he's a Psyker. It's called Maelleus Hammerhead, this makes his hands have the power to crush Adamantium and punch holes through Dreadnaughts. He ripped the arm off without trouble and impaled your shoulder. Even if he didn't rip it off and you attempted jamming your Bolter into his neck, his Errant Armor was made SPECIFICALLY to prevent that from happening with an Adamantium neck guard. No.