Given what we've seen in action on SG-1 and some of the suggestions in this thread, how about something like this?

Zat Gun
Variable Power. If used against organic beings, the Zat Gun functions as a Weapon:0. Electronics are a different matter, against which it functions as a Weapon:3.
Short Ranged. Due to the unstable nature of the rounds fired by the Zat Gun, the effective target range is limited to 1 zone.
One To Stun, Two To Kill. If an organic being is successfully hit and takes stress from this weapon, they immediately take a sticky aspect Stunned/Slowed for the remainder of the scene. If the same target is successfully hit and takes stress from a Zat Gun in the same scene, they die.

I figure the Weapon:0 against organic beings is true to the portrayal of the gun in the show. If the target has armor, it will be harder to damage and give the sticky aspect, just like in the show. I just tossed this together, so feel free to modify or critique at will.

Just FYI - the stats I am using for my character are Conviction and Discipline of 5 and Lore of 4.  My Thaum. specialization is in crafting uses +1 and I have a focus item that gives me a +1 to complexity.  So if I did my math right  - I should be able to make items that are Power 5 effects and usable twice a session.

I don't really need to make up deficits in my casting abilities, however the point of thaumaturgical effects in enchanted items is worth noting.

Actually it should be thrice per session. Exchanging one point of power or using an additional enchanted item slot grants an additional two uses.

I wanted to leave a little leeway in there, but the ambiguity isn't really a good thing. Upon further consideration, I figured this needed more fleshing out, and a means to get out of the frenzy if the target is an ally and the wielder has no FP and the party does not want to kill them.

Shooting Frenzy. When the wielder has Sponsor debt equal to the refresh spent on it, they feel an uncontrollable urge to kill. The GM may compel the wielder to attack the nearest living target with this gun. This effect may be negated by paying an amount of Fate Points equal to the current level of debt. Otherwise, the wielder will attack only this target until it is dead, or until the wielder is disarmed of this gun. If the wielder does not kill the designated target, they will be unable to access the abilities of this weapon for a number of scenes equal to half of the debt used to compel them, or take an equivalent mental consequence.

Do you think the last sentence is unfair? I don't want the player to be able to cheat their way out of not having some sort of consequences for failing to achieve the compel.

For example a lack of sexual organs can be justification for immunity to seduction.

Or the character could just have an aspect that says that they are asexual. Why pay for powers when you can have aspects for some of these things for free?

It sounds like I really need to grab that RPG!!

I don't know why I messed up that discount part, you are totally right.

With regards to Shooting Frenzy, the bullet count was kinda iffy to me already, so I can scratch that. Also, I meant for that to read as a social consequence instead of social stress (oops). If the wielder gets unnerved or really intimidated they start shooting because it makes them feel better. I specified "living creature" in an effort to make the gun somewhat treacherous to use too much lest the wielder start shooting at their allies. Given that there will be lots of undead type creatures in my campaign, I saw that as an easy way to compel them against their nearby allies.

How would you suggest implementing a debt-compel mechanic?

Here's an IoP I devised for a campaign I'm about to run. The player wanted some help building a gun that might be quasi-sentient/evil/vampiric so this is what I came up with.

The Peacemaker of Jesse James [-3]
Description: You possess the Colt .45 of the infamous Jesse James. This weapon was cursed by an unknown hoodoo doctor after Jesse James killed his father. This gun is cursed to be passed on to the firstborn of each successive generation of his bloodline.
Musts: You must be a firstborn descendant of Jesse James to inherit the Peacemaker, represented as a high concept. If any other creature wields this weapon, it functions as a normal gun.
Skills Affected: Guns, others.
Effects:
All Creatures Are Meant To Die. This is the truest purpose of the Peacemaker, to make sure that its target is deader than dirt. When facing an opponent, the wielder may take one shot (this effect may be extended to six shots if the wielder wishes to spend a fate point) that ignores that target's defensive abilities (it may satisfy whatever "the Catch" is), as well as any mundane armor it may be wearing. However, the wielder must make an immediate Discipline roll after the shot (or spend a fate point) to avoid going into a Shooting Frenzy (see below). This skill may be used multiple times per scene.
Debilitating Shot. Any time a creature is successfully wounded with this gun, the target experiences a supernatural loss of vitality. Subsequent shots made with this weapon against the same target gain a +1 on the roll for the remainder of the scene.
Killer's High. Once per scene, if this weapon inflicts enough stress and consequences to kill a living creature from a gunshot, the wielder may take an immediate "free" recovery period equal to an extra scene. This will clear a stress track and minor consequence of the wielder's choice.
Shooting Frenzy. When outnumbered in combat, after receiving social stress, or after emptying the cylinder (six shots), the wielder feels an uncontrollable urge to kill the nearest living creature with this gun. The GM may call for Discipline rolls to resist this urge. In some cases, the urge may take the form as a compel against the wielder's high concept.
It's a Gun. The Peacemaker of Jesse James always takes the form of a single action revolver with a revolving cylinder holding six rounds.
Unbreakable. As an item of Power, this gun cannot be hexed and will never jam.
Discount Already Applied. As an Item of Power, the gun already includes the one-time discount. If this is the second Item of Power carried by a character, the refresh cost is -5.

Any thoughts?