It's a trivial endeavor--if you're supernatural. For normal mortals, it requires access to military hardware, explosives, or hitting someone with a car.
What weapon you have is dependent on what the player decides to bring. If losing the weapon isn't a compel, then what, exactly, is stopping them from bringing it somewhere? If the player's put that much into the weapon--spending multiple fate points on just using it means they probably have an aspect about it, especially if it's an item of power--then it's worth a fate point for them to not have it. Why do you think Items of Power always require an aspect relating to them? Or powers having to fit the high concept? It's so that that aspect and concept can be compelled when they lose access to those powers for whatever reason, i.e., a wizard getting a compel in a deluge that his magic doesn't work, or a Faerie being unable to use his speed and toughness because he's surrounded by iron.
Nothing says so in the rules, but consider this: No two of the listed canon stunts have the same condition. The stunt rules are clear about not stacking effects--which tells me that they don't want more than one stunt applying in full to a single action. Having the weapon focus stunts means you are getting the full bonus (+1 to attack, +2 to stress), for one condition, on the same action, stacking directly.
Kincaid is hard to gauge because we never see into his head or, really, know what he's capable of. I've speculated elsewhere that he has some kind of "hunter" Supernatural Sense--maybe he can make special declarations or assessments on where to shoot things to make sure they stay down. Also, when he's one-shotting Denarians, that's with a high-powered sniper rifle from ambush--and I wouldn't put it past him to have gotten holy bullets from somewhere, come to that, or at least armor piercing rounds. So Kincaid can easily be doing it through ambush, declarations, and finding weaknesses instead of just a flat bonus to using his weapon. Or he might have a 6 in Guns normally. All in all, he's not a pure mortal and at least a couple hundred years old, so we really can't use him as an example of what mortals are capable of.
A better gauge for how pure mortals kill things with Guns is Murphy, in Aftermath--she attacks from ambush (i.e., the wizard's rolling from 0), she waits until he's in the middle of a spell (a declaration/assessment, boosting her already-high Guns stat), she's using an automatic weapon (so it's Weapon:3), she probably takes a navel-gazing maneuver or two if she has the time (I seem to remember a bit in the narration about steadying her breathing), and she unloads a burst at his head (either a declaration for a +2, or maybe a stunt with the attendant downside of going through ammo quicker). Her listed Guns skill is 4, but it's safe to presume she's boosted it, meaning it's her 5, +2 for the declaration, +3 for the type of weapon, +2 for navel-gazing, and +2 for the burst. Against a 0 roll, that's a solid 14 shifts of stress right out of the gate, way more than enough for a kill in a one-shot adventure like that where the odds are already stacked against Karrin and her player, even if the badguy's got Supernatural Toughness.
For the ghouls, that's the thing--the ghoul has also spent refresh on fighting. More refresh than a pure mortal at the lowest level has, as I recall.
By "spending fate points" I mean doing things like making declarations, boosting an individual roll, and invoking scene aspects. To my reading, stunts aren't supposed to replicate powers in form or function--there's not supposed to be a "family" of stunts with requirements and stacking bonuses, so much as a stunt is supposed to be a particular situation in which the character has a slight edge.
