Disarming using a firearm

[jimofpeace]

How would you shoot a weapon out of a person's hand using the 7th edition rules?

[Kyrral]

I don't know if there is any way to really do that within the rules specifically persay, but here's what I would suggest:

Given that a combat maneuver is used to disarm normally (using fighting(brawl) or fighting(sword) for example), consider using the firearms(rifle) or other applicable weapon in place of that. I would oppose that skill vs the targets STR(Grip), DEX(reaction speed to impact) or possible POW(willpower, or determination to keep hold) then go as the following.

Success: A simple disarm. No damage.
Hard Success: A disarm plus minor damage (1d3 at most)
Extreme Success: A disarm and malfunction of that weapon, if applicable, or maybe knocks it across the room, plus maybe normal damage?
Critical Success: The weapon is both disarmed and sundered (broken), and damage applys as normal.
Failure, Fumble, or Critical Failure: Something appropriate. Maybe instead of disarming them you drop your weapon, or on a critical failure accidentally hit an ally behind your target, or maybe a ricochet. Of course, weapon malfunctions or jams would work as normal. 

A penalty die or bonus die may apply depending on the calibur of gun being shot, size of the weapon being disarmed, whether or not the weapon is being wielded two-handedly or not, etc. As always, just try and keep it fair.
Typically, characteristic values are higher than Fighting or Firearm skills, so succeeding in the first place is likely difficult. A 45 Firearms(rifle) vs a 70STR isn't exactly easy. Giving the opposed player a choice of STR, DEX, or POW should ensure that the recipient holding the weapon should have the upperhand, which makes sense. 

[soltakss]

The way I normally do it in BRP-style rules is to apply a penalty to the shot (Hard/Very hard) - Success means disarmed. I don't have a STR vs STR roll as it is the shock/impact that disarms.

[el_octogono]

I would add one or two penalty dice. I think they are useful for a fast and safe way to manage these situations. 

[Mike M]

Depending - 

in straight Call of Cthulhu  - probably an Extreme difficulty shot with one or two penalty dice (dependent on size of gun being shot at)

in Pulp Cthulhu - probably an Extreme or Hard difficulty shot