These rules are a work in progress so hopefully expect a second part with more stuff.
Edit: Thanks to Xiw on #tinypewtermen (Irc.synirc.net) for some mistakes I made and some inspiration and opinions that got me to tweak and improve my initial version!
Gamma World House Rules Document
Healing Surges: Once per 24 hours including at least 6 hours of rest outside of combat players may heal all HP damage. In combat once per day PCs may heal ¼ th (rounded up) HP damage as a Standard or Move action. At level 4 and 8 PCs gain 1 extra in Combat ¼ HP surge.
Other Effects: Negative effects that are normally “Save Ends” now stay on the target but may be avoided as normal using a Combat surge or automatically removed via the Full option.
(Note special abilities or items can allow for extra saves or surges as well.)
Weapon Reloads: Weapons that have ammo now have a “Reload” action. Reloading a weapon is either a Move or Attack action. This does mean you can shoot your guns as much as you want now.
(Optional: weapons with ammo have a condition. Every extra time it is fired roll a D10. If the roll equals or beats the condition points of the weapon it degrades by 1 point. Repair is then needed.)
Weapon Range: Weapons now have a range. At Short the weapon's hit modifier is added to your roll. At Medium it is not. At Long the roll is now -5.
Aimed Shots: Aimed Shots are an extra -5 to hit but if successful are counted as a critical hit (max damage) or if something with a weak point or multiple locations (like a Romero styled zombie say..) that is hit and rolled for damage as appropriate.
Critical Hits: A normally rolled Critical Hit also counts as an Aimed Shot as the firer's choice.
Weapon Special Abilities/Upgrades:
Indirect Fire: Indirect Fire shots are 15 AC if target/target square is visible, 25 if not visible. When the hit is a miss, for every point missed the shot 1 square in a random direction, ½ if going back towards the firer. (Round up) Scatter roll is 1d10 with 1 back towards firer, numbers go clockwise with 10 being a “dud” shot. When a Hit is rolled the shot hits the target square/target but it scatters d4-1 if indirect. (Again, a 10 rolled for direction is a dud even if you scatter 0 range on an Indirect spot.)
Cone Attack: Cone Attacks ignore cover bonuses for Hit rolls. Range is distance weapon fires. Shot is reduced 1 die in damage per square after the second. No Long/Med/Short ranges.
Multifire : May not be used at Long Range. Requires weapon to be held 2 handed. Uses multiple rounds of ammo. (Roll 1d4 shots used) If main target is hit, any adjacent targets take half damage regardless of AC. Target's cover AC is reduced by 1 step. (Partial is cancelled, Full becomes Partial.)
Multifire weapons may be fired on “Single” shot mode but lose all multifire bonuses and penalties. (So you no longer waste multiple shots or have to fire 2 handed unless the weapon itself requires it. And you know, you get Long Range!) As long as you have 3 rounds in the weapon you can still go Multifire and if you roll a 4 for shots just count it as having been 3.
Antitank (#): Antitank weapons ignore DR equal to the (#) value.
Heavy: Heavy weapons may only be fired if the firer has not moved at all including Shifting. If the firer does move it may be fired at half normal range values and it is -5 to hit.
Melta: Melta weapons are ½ normal weapon type range with Antitank +5 and ½ normal ammunition, rounded up. Stacks with Antitank.
Laser: Laser weapons are +2/4/8 range, +1 To Hit, ½ ammo capacity rounded down.
Flamer: Cone Attack doing Fire damage. No To Hit rolls required but targets evade on a 11+ roll with their Dex bonus added to the roll. Targets hit take 5 Ongoing Fire, Save Ends. No Cone Damage Dice reduction. All targets in a straight line take the hit, adjacent squares to the line take half damage with no Ongoing Fire.
Red Dot Sight: +1 To Hit rolls, target may either use AC or Reflex, whichever is higher.
Sniper Scope: If firer doesn't move, weapon gets +2 to hit and +4/8/16 to range.
2 Handed: Requires 2 hands to fire, otherwise -5 to hit.
Blast (#): Hits target square full on, surrounding squares take ½ damage. (#) is size of the blast.
Light: Range: 5/10/20 Damage: 2d4 Ammo/Clip: 9 To Hit Bonus: +3
Medium: Range: 7/14/28 Damage: 2d6 Ammo/Clip: 7 To Hit Bonus: +2
Heavy: Range: 8/16/32 Damage: 3d4+1 Ammo/Clip: 6 To Hit Bonus: +1
Carbines: (ALL 2 HANDED)
Light: Range: 9/18/36 Damage: 2d6 Ammo/Clip: 24 To Hit Bonus: +3
Medium: Range: 11/22/44 Damage: 2d8 Ammo/Clip: 20 To Hit Bonus: +2
Heavy: Range: 13/26/52 Damage: 3d6+1 Ammo/Clip: 16 To Hit Bonus: +1
Rifles: (ALL 2 HANDED)
Light: Range: 15/30/60 Damage: 2d8 Ammo/Clip: 8 To Hit Bonus: +3
Medium: Range: 20/40/80 Damage: 2d10 Ammo/Clip: 6 To Hit Bonus: +2
Heavy: Range: 23/46/92 Damage: 3d8+1 Ammo/Clip: 4 To Hit Bonus: +1
Machine Guns: (ALL 2 HANDED, HEAVY, MULTIFIRE)
Light: Range: 25/50/100 Damage: 2d10 Ammo/Clip: 20 To Hit Bonus: +3; Antitank 2;
Medium: Range: 30/60/120 Damage: 2d12 Ammo/Clip: 16 To Hit Bonus: +2; Antitank 3;
Heavy: Range: 32/64/128 Damage: 3d12+1 Ammo/Clip: 12 To Hit Bonus: +1; Antitank 4;
Sawed Off: Damage: 4d6 Ammo/Clip: 2 To Hit Bonus: +1; Cone 5
Hunting: Damage: 5d6 Ammo/Clip: 2 To Hit Bonus: +2; Cone 7; 2 Handed
Combat: Damage: 6d6 Ammo/Clip: 8 To Hit Bonus: +3; Cone 9; 2 Handed
Slug Ammo: Shotguns may be fitted with “Slug” rounds that turn the Cone range to Medium, with Double that Long, and NO Short range. Slug rounds fire like normal guns and ignore Cone rules. Slug shots are also -2d damage dice. (So a Combat Shotgun with Slug would have -/9/18 for range, and do 4d6 damage/
Incendiary Ammo: As Slug but damage dice is HALVED from the normal Shotgun damage, and the shot is Fire damage, adjacent targets take half damage. 3 Fire Ongoing, Save Ends.
Light: Damage: 3d6 Ammo/Clip: 10; Flamer Cone 6; 2 Handed
Heavy: Damage: 3d8 Ammo/Clip 6; Flamer Cone 10; 2 Handed
Heavy Weapons: (ALL 2 HANDED, HEAVY)
Grenade Launcher: Range: 10/20/40 Damage: 2d6 Ammo/Clip: 6 To Hit Bonus: +1; Indirect Fire; Blast 2
(ADD SPECIAL GRENADE TYPES)
Bazooka: Range: 18/36/72 Damage: 2d10 Ammo/Clip: 1 To Hit Bonus: +2; Antitank 5
Box Launcher: Range: 20/40/80 Damage: 2d12+2 Ammo/Clip: 4 To Hit Bonus: +1; Antitank 10;