Looking at the Wounds Table, and at places like FUG 42, it appears that "disarming" is simply another attack, one that gets a Hurt result (no armor). So, if you declare you are trying to disarm your opponent as part of a combat exchange, that's what you roll to try to hit.

If you try to disarm someone with a ranged weapon (shooting someone's gun out of their hand), they get a Superior Weapons bonus in the exchange.

The Hurt result (see Wounds Table) isn't actually persistent. It's a "Hurt-equivalent", basically robbing them of a weapon (though they might have another one to pull out of the bag). [1] Depending on the circumstance, you could give a Clipped to indicate the (one exchange) discomfiture and disorientation of having to change combat modality and tactics.

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Combat Rules