Weapon & Combat Rules with a Touch of Crunch

A short bit of combat crunch I've been tinkering around with; these are mostly unplaytested. Armor is yet to be determined, and Jason's post about armor might well upend a lot of this anyway. These rules are pretty flexible as far as armor is concerned, anyway.

These rules assume you're playing a game with d20 to-hit rolls, variable damage dice ranging from 1d4 to 1d12ish, and either use a grid or a grid-like mentality. I'm assuming ascending AC and higher-is-better attack rolls, but it'd be pretty easy to reverse either of those.

GOALS WITH THESE RULES: 

1. Express that a spear will beat a knife or sword at long range, but once the smaller weapon is closer, the spear will lose.
2. Express that in tight quarters, a really big weapon, like a zweihander or maul, will struggle.
3. Make it easily-hackable into most OSR systems.
4. Do it all without adding significant new dice rolls, item tags, or hefty mechanics.

OK, onto the proper mechanics:

Weapon Variables

Weapons have three basic variables:

1. Handedness; how many hands it takes to wield (one or two, typically)
2. Damage; how much damage the weapon does (1d6 -> 1d12, generally)
3. Reach; how long the weapon is, and how far away you can attack with it

For example, a hatchet is a one-handed short weapon that deals 1d6 damage; a maul is a two-handed midrange weapon that deals 1d12 damage; a spear is a one-handed long weapon that deals 1d6 damage.

Assigning Damage Dice

Generally, the damage dice of a weapon is as follows:

• Short and long one-hand weapons deal 1d6 (e.g. knives, spears, hatchets, maces) 
• Midrange one-hand weapons deal 1d8 (e.g. swords, warhammers, flails)
• Long two-hand weapons deal 1d10 (e.g. pikes, zweihanders)
• Midrange two-hand weapons deal 1d12 (e.g. mauls, greataxes)

Versatile Weapons

Any weapon normally wielded with one hand that has the physical holding-space to be wielded with two (like a battleaxe with a hilt or sword with a ricasso) can be wielded with two hands, which increases the damage die by one size (e.g. 1d8 -> 1d10).

Improvised Weapons

If you’re fighting with something that isn’t normally meant to be a weapon (such as a table leg or broken bottle) or using a weapon in a non-conventional capacity (such as beating someone with the hilt of a sword), reduce the damage die by one size (e.g. 1d8 -> 1d6).

Reach and Initiative

When combat starts, the combatant with the longer weapon has initiative: they get to go first. This continues until the other combatant scores one damaging hit, at which point they gain the initiative; this usually means they take two turns in a row as the initiative order reverses. 

Long weapons have an additional modifier: when they have initiative, they have advantage on attacks, but when they don’t, they have disadvantage. They also probably have additional reach in combat, usually an extra 5 ft.

If combatants have weapons with the same reach, they act simultaneously. This means they can—and sometimes will—kill each other.

Close Quarters

If you are wielding a two-handed weapon that needs to be swung (like a maul, but not like a spear), you suffer -2 to hit per side of your square that’s occupied; it doesn’t matter what occupies it, it can be a friend or enemy or object, whatever. If the ceiling is exceptionally low, that counts as another occupied square, too.

However, this doesn’t apply to small weapons; they can attack freely in any environment, and never suffer penalties for tight spaces.

If you choose to, you can wield your weapon in such a manner as to reduce its size (such as holding a maul closer to the head, or half-swording with a longsword). When you do so, for each size you reduce the weapon by, reduce the weapon’s damage die size by one, as if you were using an improvised weapon.

If, for example, you’re fighting with a zweihander with your back against a wall (one side), you suffer -2 to attack rolls; if you’re fighting while backed into a closet (four sides, including the ceiling), you suffer -8. However, if you decide to half-sword that zweihander, thus reducing it to a small weapon, you would suffer no penalties to hit, but would only be dealing 1d6 damage, rather than the standard 1d10.

Flanking
If you are being attacked by more than one attacked, each attacker gains +2 to hit for each additional attacker after the first, provided they are attacking from multiple directions.

If, for example, you are attacking a monster and your comrade is attacking it from behind, both of you have +2 to hit. If you are surrounded by armed guards on all four sides, each of those guards has +6 to hit you.

Soft Power
Remember that weapons have more utility than simple damage; they have other uses and capabilities. I call these extra, less-strictly-mechanical uses "soft power." An axe, for example, can easily cut down a tree or chop through a door, where a sword or mace will struggle to do so. Likewise, hooked weapons can catch ropes and levers, heavier weapons can be used to bar doors, and small ones can be hidden in boot-legs.

Furthermore, consider the social aspect: someone walking down a street with a knife in their belt will draw little notice (at least in a fantasy context), while someone carrying a greataxe will usually send citizens running for the guards. In an even fight, military-grade heavy weapons are nearly always preferable, but adventurers are almost never in an even fight.

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It's entirely possible these rules seem obvious and/or unnecessary to you, which is fair. I'm honestly mostly writing this for my own (and my players') benefit.