Angst driven characters can be interesting but sometimes a bit one dimensional but so long as his suicidal thoughts become an obsession

The fact that he's lean an in shape is of small consequence because let's face it in the World of the Walking Dead after 2 years only the strong or subjugated survive.

I do however have issue with a fully automatic weapon this early in play
Also I will point out that arming a weapon like an AK-74 which uses an odd caliber round ( the 5.45×39mm) would mean Brass much less actual loaded rounds would be difficult to come by
A Gun smith is useful but pick whether he's good with using or repairing weapons
I am very familiar with guns an gun smith. I have found at Jacob's age he'd be one or the other but seldom both
By the above I mean he's an expert at Repair and a fair shot or he knows the mechanics of guns enough to keep them working and is an expert shot with almost any type
Both of the Above skills require extreme dedication and
Splitting your attention lessens their effectiveness

Oh and the Most common round found in any gun store is the 22, 223 then the 308, an 30.06 so I'd stick to one of these calibers as they are the easiest to scavenge