hm. I'd treat it as a Bionic, but also make bionics [and any unarmed and natural weapons that aren't power-fielded or warp weapons] vulnerable to a Stormfield-style burn when parried. Same chance of rolling damage to that location as there is of 'destroy weapon'. The "bionic" new "limb" gives +2TB to its owner for this of course.
We can let using parry WITH them imply you're stopping them at the forearm, hand or handle of the weapon, but things get rather ridiculous when the guy with the giant glowing sword slams it down "defensively" on your tentacle and somehow nothing happens. Same wierdness occurs in Star Wars RPGs, when successfully blocking a headbutt with a lightsaber fails to so much as singe the one that tried it.
So here's how I'm thinking:
Weapon Rules: A Toughness Test [Challenging] is allowed to avoid Jams and Overheat when they occur. If successful, the weapon operates normally, avoiding the effect entirely [though most likely misses with such a roll]. They otherwise function normally. While less likely to occur, its still a chaos-affected weapon, and its stuck to you. Try not to explode your arm off.
Unnatural Limb: The melded limb is now a bionic [+2TB]. Hits applying to it treat it as either having the Natural Weapon [melee] or Machine [ranged] Traits for purposes of damage effects. Effects that damage or disable the forearm or hand affect the weapon arm in the same way: it requires healing or outright regrowth/regeneration/cloning/whatever to recover. This may be very difficult to obtain but the Dark Gods have seen fit to inscribe the weapon's compatibility in your genetic code. Just get yourself a new copy of the same weapon [model, pattern, quality], medicae will do the rest as you heal up.
Feed Upgrades: modifications to the weapon's firing such as Twin-Link, Storm, Single-Shot Downgrade, Belt-Feed, etc remain, but further such modifications cannot be applied to a Slayer Arm. A weapon with Fire-Selectors retains a single additional magazine slot, but may replenish it as a full action for one level of fatigue.
Handling Upgrades: Stock, Trigger and Grip upgrades are lost and may not be further installed. You no longer have a handle, and that unfortunately tends to diminish the movement range of the weapon. No chance of a Custom Grip
Barrel, sight and most miscellaneous upgrades can be installed normally, with the normal limits.