"The necessary adjustment would be to make certain the rifle accepts the ammunition I craft for it. I can preserve its ability to channel the caster's energy directly. As to the prevention, it would simply be a matter of allowing the passphrase to be used only by those attuned to it."

And to prevent someone from grabbing it effectively disarming the person by using the passphrase to dismantle it. Call Elidibus a worst-case scenario planner.

"...When you say harm, do you mean the prevention or the ammunition? And if it's the latter, I am curious about your reasoning."