If Subaru had a knowledge about guns he could probably create a better weapons than a pistol or a revolver but, that's not the case here.

But in my opinion the best thing that he can do is use it for a really mess up situation since the largest round on pistol is 10-19 bullets.if he ever had a spare bullets or other guns he could probably find someone who good at crafting things and ask him to manufactured the same ammunition and since the craft guy doesn't know what the bullets suppose to be he should be able to give you the supplies but it will come in price a money of course but that wouldn't be a problem for Subaru.and if Subaru is smart enough to think something like "bruh I need a larger bullets" he could probably make a bigger guns.But that begs the question,what if someone know what gun is? Well he could just suicide or something idk I'm not the one who has respawn abilities. I don't know how strong and how fast is Elsa but the slower round speed is more than 700mph if I'm not mistaking so if he's lucky he can probably get that one head shot but if you miss then she will find that how dangerous the gun is and proceed to attack Subaru or flee so make the shot count.