I wrote this because my players missed the excitement of fugking up good, only with firearms (we used to play D&D where fighters had a tendensy to throw their swords away on natural 1s, or hit nearby buddies - they wanted that in d20 Modern too). These rules are supposed to be compatible with my new system (coming soon!), so you might want to replace Engineering with Craft (mechanical). The Reliable/Unreliable thing is inspired from D20 Mecha SRD and the Ultramodern Firearms book (if you need more info on either, ask me, or google it).
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A not Unreliable weapon rolls on Table 1: Firearm Fumbles when the character rolls a natural 1 on his attack roll.
An Unreliable weapon that rolls a natural 1 or 2 on the attack roll has a 50% chance of agravating the fumble to a Critical Fumble. Thus, a character who fumbles with an Unreliable weapon who rolls the Failure to Feed fumble must roll a d100 to see whether it becomes a Critical Failure to feed.
A Unreliable x2 weapon that rolls a natural 1, 2 or 3 on the attack roll, has a 50% chance of agravating the normal fumble to a Critical Fumble, and a 50% chance of agravating the Critical Fumble to an Improved Critical Fumble.
For every Unreliable drawback added to the weapon, the Fumble range increases by 1, so an Unreliable x4 weapon will have a Fumble range of 1-5. Very Unreliable weapons even risk destroying the entire weapon (see Table 4).
A character that rolls a natural 1 with a Reliable weapon must first roll a d100, results below 50% means that the weapon simply missed, and results above 51% means that the PC will now be rolling on Table 1: Firearm Fumbles. For every additional Reliable quality the weapon has, there's an additional 10% chance that nothing happens (thus, a Reliable x3 weapon will have a 70% chance of ignoring the fumble, and a Reliable x6 weapon will never need to roll on the following tables). It should be remembered that some things will cause a weapon to gain the Unreliable drawback (for every Unreliable drawback a Reliable weapon gets, simply substract 1 Reliable quality until there's none left, and then the weapon starts becoming Unreliable).
Notes: If certain parts of the weapon are damaged or destroyed (for example on Table 3), having another part (such as a new firing or feeding mechanism), will reduce the time by 75% and the DC by 8. For a result on Table 4, where the whole weapon has been damaged, each replacement part (either the feeding, the firing, or the ejecting mechanism) reduce the time by 25% and the DC by 3 (thus, having all the replacement parts will reduce the time required to a bit over one hour, and the DC to 20).
A character without the appropriate weapon proficiency takes -10 to any check relating to clearing or repairing the weapon(s). A character with Weapon Proficiency and Weapon Focus in the weapon gains +5 to any check relating to clearing or repairing the weapon(s).
Table 1: Firearm Fumbles
1-25% - Failure to Feed: Often due to a faulty magazine or a bad feeding mechanism, this requires a re-loading of the magazine, which follows the normal Reloading rules. If using an Unreliable weapon, there's a 50% chance that it becomes a Critical Failure to Feed (see Table 2).
26-50% - Failure to Fire: Often due to faulty or exotic ammunition, this requires a removal of the round in question, which is a move action and requires a Dex check DC 10 which provokes attacks of opportunity. If using an Unreliable weapon, there's a 50% chance that it becomes a Critical Failure to Fire (see Table 2).
51-75% - Failure to Eject: Often due to the shell casing getting stuck in the ejection mechanism, this requires a full round action (which provokes attacks of opportunity) and a successful Dex check against a DC of 12. It should be noted that this is the only case where the weapon actually fires (except the Nothing Happens case, below). If using an Unreliable weapon, there's a 50% chance that it becomes a Critical Failure to Eject (see Table 2).
76-100% - Nothing Happens: You're were lucky, you simply missed your target. If using an Unreliable weapon, reroll on Table 2 instead. If using an Unreliable x2 weapon, reroll on Table 3 instead. If using an Unreliable x3 or more weapon, you get the result of Table 4.
Table 2: Critical Firearm Fumbles
1-33% - Critical Failure to Feed: Magazine is unusable and must be replaced, either due to a production fault, or weapon abuse. If using an Unreliable x2 weapon, there's a 50% chance that it becomes an Improved Critical Failure to Feed (see Table 3).
34-66% - Critical Failure to Fire: The round has jammed in the firing mechanism, and cannot be cleared without an hour's work and a successful Engineering Skill check against a DC of 14. If using an Unreliable x2 weapon, there's a 50% chance that it becomes an Improved Critical Failure to Feed (see Table 3).
67-100% - Critical Failure to Eject: The round has jammed in the ejection mechanism, and cannot be cleared without an hour's work and a successful Engineering Skill check against a DC of 17. If using an Unreliable x2 weapon, there's a 50% chance that it becomes an Improved Critical Failure to Feed (see Table 3).
Table 3: Improved Critical Firearms Fumbles
1-33% - Improved Failure to Feed: The feeding mechanism is destroyed, and cannot be repaired without two hour's work and a successful Engineering Skill check DC 20. If using an Unreliable x3 or more weapon, there's a 50% chance that it becomes an Advanced Critical Failure (see Table 4).
34-66% - Improved Failure to Fire: The firing mechanism is destroyed, and cannot be repaired without two hour's work and a successful Engineering Skill check DC 22. If using an Unreliable x3 or more weapon, there's a 50% chance that it becomes an Advanced Critical Failure (see Table 4).
67-100% - Improved Failure to Eject: The ejection mechanism is destroyed, and cannot be repaired without two hour's work and a successful Engineering Skill check DC 24. If using an Unreliable x3 or more weapon, there's a 50% chance that it becomes an Advanced Critical Failure (see Table 4).
Table 4: Advanced Critical Firearms Fumbles
1-100% - Total Failure: The whole weapon is damaged, and the user must succeed a DC 18 Reflex or take 1d6 points of damage (the GM decides what kind) as parts may fly off. Five hour's work and a successful Engineering Skill check DC 29 is required to make it usable again.
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Keep in mind that I don't know much about firearms, I've simply read some articles about firearms malfunctions, and have based this on them. Any critique appreciated.