The D&D Master Set introduced a weapon proficiency system called "weapon mastery".
The system is definitely a game-changer, taking BECMI completely away from B/X with powerful combat options for players (and their opponents!) and greater tactical complexity.
Running combats becomes a lot more fun for seasoned players, although PCs will punch well above their weight (roughly 3-5 levels per increased level of mastery, depending on class).
Limited Weaponry:
When a human character is started, the player is limited to selecting only two weapons for that character’s use.* Knowledge of one additional weapon can be attempted when each of the following levels is reached: 3, 6, 9, 11, 15, 23, 30, 36.
*The DM may allow fighters to start with three or four weapons instead of two. As a further option, a fighter may be allowed to also add a weapon at levels 19, 27, and 33.
Because of their longer lifespans and wilderness-oriented lifestyles, demi-humans start with Basic skill in all weapons not forbidden to them. Training is possible only at levels 4 and 8 (and, for dwarves, level 12) and each 200,000 XP after.
Weapon Mastery:
A character may decide to train to become more skilled with one weapon currently used instead of learning a new weapon. The amount of skill is called the level of mastery.
A starting character can have only Basic knowledge of the starting weapon(s). The levels of mastery are: Basic, Skilled, Expert, Master, and Grand Master.
Attack Benefits:
Damage increases with each level of mastery, and can vary by the type of opponent.
Opponents who attack with two-handed missile fire devices have similar defenses to those who attack with natural body weaponry (claws, teeth, etc.). This type of opponent is designated “M” (Missile/Monster).
Opponents using weapons held in the hand and swung or thrown (including sling and bola, but excluding all other missile fire devices) must defend very differently. These opponents are designated “H” (Hand-held).
Defense Benefits:
Increasing levels of mastery confer a bonus to the Armor Class of the defender.
The AC bonus applies only to a given number of attacks per round. The use of AC bonuses does not count as an action; the character may still move and attack at normal rates.
Special:
Charge - if the user charges 20 yards or more and strikes his target, the victim takes double damage.
Death - the victim is reduced to 0 hit points.
Deflect - in addition to any attacks, the user of this weapon may attempt to deflect the number of melee and thrown weapon attacks indicated in one round. To deflect each attack, the character must make a Saving Throw vs. Death Ray.
Delay - the victim must make a saving throw or lose initiative the next round. If a saving throw is not specified, make a Saving Throw vs. Paralysis. For missile attacks, this effect occurs only at the indicated ranges.
Disarm - the user may attempt to disarm an opponent instead of making a normal attack. The attacker must roll to hit the target. The victim may save by rolling less than or equal to his Dexterity on ld20. If the attacker is normally able to disarm as a fighter, the victim must save at a +5 penalty. Dexterity scores for NPCs and monsters should be determined by the DM, or else treat as 11. For each level of mastery the attacker has gained beyond Basic, the victim suffers a penalty of +1 to his saving throw vs. the effect.
Double Damage - on a natural roll of the numbers indicated, the weapon inflicts double damage.
Entangle - an entangled victim cannot attack, cast spells, or move. The victim may make a Saving Throw vs. Death Ray each round to escape.
Hook - instead of making a normal attack, the user of this weapon may attempt to hook and pull down a foe. The attacker must roll to hit. This causes minimum damage, and the victim must make a Saving Throw vs. Paralysis or fall down. A +4 bonus to Hit rolls applies when attacking a fallen foe. A fallen foe also has a -4 penalty to all saving throws and a -2 penalty to Hit rolls while on the ground. It takes one round to stand up.
Ignite - fire has a chance to ignite anything flammable equal to 5% per point of damage caused in each round. If an item ignites, it will bum for 1-6 rounds causing 1-4 points of damage each round.
Knockout - the victim is rendered unconscious for 1d100 rounds.
Paralyze - there are two forms of paralysis. The first type freezes a creature in place for 1-6 turns. A paralyzed creature is conscious and aware of what is happening but cannot move, attack, talk, or cast spells. The second type of paralysis results from a failed strangle attempt, leaving the victim too weak to move for 2-12 rounds. Attacks against a paralyzed creature automatically hit. Any cure spell may be used to negate the effects of paralysis but does not cure damage in addition to the paralysis.
Second Attack - the user is able to make a second attack with the shield weapon while attacking with another one-handed weapon.
Set vs. Charge - if the user is aware of a charging enemy, the weapon may be set against the charge. If the weapon hits the charging foe, it inflicts double damage.
Slow - the victim is slowed and can move and attack at only half the normal rate. No spells may be cast while slowed.
Skewer - the user of this weapon may decide to skewer a foe rather than making repeat attacks, if the foe has no more than the number of Hit Dice noted. The maximum possible Hit Dice for the purpose of this attack is 9. Once the weapon hits, it is stuck in the foe and cannot be removed for 5-8 (d4+4) rounds. For each round a victim is skewered, he will automatically take 1-6 points of damage.
Strangle - if the attacker makes a natural roll of the numbers indicated, the victim must make a Saving Throw vs. Death Ray or be immediately paralyzed and die in 3-8 (d6+2) rounds unless rescued. If the victim escapes, he remains paralyzed for 2-12 rounds.
Stun - if the victim is approximately the same size as the attacker or smaller, he is stunned if he fails a Saving Throw vs. Death Ray. A stunned creature moves at one-third speed and cannot attack or cast spells. The victim also suffers a +2 penalty to his Armor Class and a -2 penalty to all saving throws while stunned. A Saving Throw vs. Death Ray may be made each round to recover from the stun effect. For missile weapons, this effect occurs only at the specified ranges.
Unskilled - if Unskilled with a weapon, a user attacks at Basic mastery level but inflicts only half damage. All missile weapons have a penalty of -1 to all Hit rolls.
% hp - the victim loses this percent of his original hit points.
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