As with any rules set, you want something that will appear realistic without being too complicated. The particular problem with creating a disarm mechanic, or any mechanic that deals with special combat tactics or maneuvers, is you want to create options that can be attractive and useful (and thus meaningful) but not so attractive and useful that they completely disrupt or unbalance the general combat mechanic.
Let me offer a suggestion (the following is based on a “first three booklets only” OD&D rules set—thus hit dice are 1d6, among other things—and I will use 16th-century English weapons in one of the examples, but I think it is easily adaptable):
After a successful hit, but before rolling damage, the attacker may elect to try to disarm his opponent. If the defender is fighting with two weapons, the attacker may choose which weapon to target.
In my view systems that require maneuvers to be declared before rolling to hit add an extra level of unnecessary fussing. In addition, many if not most “hits” in D&D are not actual weapon strikes but rather successful attempts at getting the advantage of the defender or wearing him down. Thus, it seems appropriate that a disarm attempt would follow a hit rather than preceding it.
One might object, I suppose, that the fact that someone is wearing, say, plate armor shouldn’t make it easier to potentially disarm him. I would say three things in response: 1. Combat in D&D is abstract. That someone is wearing plate, makes them less vulnerable to actual strikes but also perhaps makes them more potentially effective at stopping their opponent from doing various things—their armor means they don’t have to worry as much about, say, using their arm to block a blow (or block a successful disarm). 2. I’m not sure I want a disarm mechanic that would make disarming the guy in plate armor the default attack (since its tough to get at him any other way). It might be that this would be somewhat realistic, but I think it would change the general combat mechanic in a bad way. 3. It’s simpler.
The base chance to disarm is 11 on 1d20 (50%) with these modifiers:
-1 for every 3 hit points of the defender (what he has remaining, not his total).
This tracks two things: It’s harder to disarm powerful characters. It’s easier to disarm anyone when they are fatigued. The hit dice (though not hit points) of the attacker are taken into account in the initial to hit roll. Again, the -3 is based on the “pre-inflationary” original three-booklets. If one is using a Greyhawk, Classic or AD&D sort of mechanic, one could make it -1 for every 5 hit points.
+/- 1 for the difference in weight between the attacker’s weapon versus the defender’s weapon.
Given the weapon weights in my game, this is not as potentially radical a modifier as it might seem. In my view, the weapon weights in all versions of D&D are too high. For example, a two-handed sword is pegged at 15 lb. in Men & Magic and 25 lb. in 1st Edition AD&D. A halberd is pegged at 15 lb. and 17½ lb., a longsword at 5 lb. and 6 lb. But I would make these weights much lower—8 lb. for the two-handed sword, 7 lb. for the halberd and 3 lb. for the longsword. I believe this is more historically accurate. See the excellent What Did Historical Swords Weigh? at the excellent ARMA site. Or go to your local Target, buy a 25 lb. container of cat litter and imagine yourself effectively wielding something that heavy.
I understand that some rules sets choose not to track weapon weight. I think this makes sense in terms of the encumbrance mechanic (I like and use James Raggi's system in his Lamentations of the Flame Princess), but I think tracking weight can have value for other purposes--such as disarm rules. If keeping track of weapon weight bothers anyone, then one could just have the referee declare a modifier on the fly ("using a two-handed sword to try and dislodge that short sword? You get a -5")
+4 if the attacker is wielding a weapon that is designed to be effective at disarming. In my setting this would include the black-bill and forest bill—polearms with all sorts of nasty hooks and curves on their blades.
-4 if the defender is wielding a two-handed weapon.
If the disarm is successful, the weapon will fly in a random (1d12) direction and land 1d20 feet away.