The "introductions" section may not be the best place to post a question, to gain the answers you seek.
Of course, reality has never stopped me before. Why should I let it stop me now?
I feel that you have posed an impossible-to-solve problem.
You have set up a situation in which your protagonist will necessarily suffer a great deal of damage, and may even be killed—or converted into a werewolf himself, by means of death.
Fifteen feet is no distance at all, to an animal which can spring at a potential victim. If the fight is not stopped instantaneously, the werewolf wins. But your parameters require that the werewolf take minutes to become incapacitated. Thus, your protagonist must necessarily lose.
So the choice of which handgun in what caliber becomes a meaningless one. No matter what the final choice, the werewolf still wins.
Why are the bullets merely to be "coated in silver"? It is much, much easier to make solid-silver bullets in the first place. Just ask The Lone Ranger—or Tonto, who, presumably, did all of the scut work for the racist oppressor of the long-suffering Red Man.
Silver melts at a temperature low enough to permit bullet casting with the normal equipment we've all used to make lead bullets. Any ammunition-reloader could do it easily.
Would use of solid-silver bullets make a difference?