I've looked high and low and found very, very little.

I reckon your best bet is to train in metal work/machining, as in knowing how to use a lathe properly ect.. and go from there, as I'd imagine rifle makers would prefer someone competent in machining before having to teach them to actually make rifles and parts.

Personally, and hopefully, after I've finished this year of college im going to do an apprenticeship as an Instrumental Technician (Already got a sponsor company )and at the same time take machining courses or similar, then just see what happens, at least that way I have a decent trade to fall back on/support myself.