A rail gun isn't a bad choice I guess, you would have to hit a vital organ though or else it just penetrates random flesh. He did mention magic being harder to cast with one hand only so this isn't all too bad.
He made some cool guns and all but they're still relatively straight forward mechanical objects he enhances with the magic/skills he got from monsters. An artificial arm that's of any use and doesn't just dangle from his stump would probably require some specialized designs and a particular skill I guess? Maybe he could do it right now if he used some for of transmutation to move it around but that sounds like it'd kind of keep his attention to it occupied a lot of the time, maybe it'd prevent him from using other skills at the same time?
I feel like he definitely will have either an artificial arm at some point or kind of go a bit monstery with a new acquired skill and regenerate. Of course, if I were a writer and gave a character regeneration long after they lost a limb and it had already healed over and all I'd say "Too bad, took too long," but I dunno what this author will consider acceptable or not.
why doesnt he just eat a part of yue that would give him regeneration abilities as well as magic affinity and yue will simply regenerate again. its a win win situation the way i see it.
Anti-Material Rail Guns being shot with one hand must be a bitch to aim properly...Then again it is a larger target so its less of an issue as opposed to the recoil of the thing.
Schlagen: to hit something. For a railgun, which is probably the most advanced method of throwing a rock that planet Earth will ever see, it's very fitting.