I'm very happy with my group's custom misfire table for firearms. We play 3.5, and whenever someone rolls a critical failure with a firearm, we roll another D20 for the misfire table. Depending on the type of firearm, there may be a greater or lower chance for a horrible mishap.
Misfire table (D20)
[TABLE="class: grid, width: 700, align: left"] [TR] [TD]Dueling pistols[/TD] [TD]Flintlock/doglock/
snaplock pistols[/TD] [TD]Multi
barrel/shot[/TD] [TD]Hold out
Pistols[/TD] [TD]Roll Result[/TD] [/TR] [TR] [TD]1[/TD] [TD]1[/TD] [TD]1-2[/TD] [TD]1-3[/TD] [TD]The firearm explodes, dealing 1d6 damage on the wielder.[/TD] [/TR] [TR] [TD]2-4[/TD] [TD]2-5[/TD] [TD]3-6[/TD] [TD]4-7[/TD] [TD]The weapon’s firing mechanism breaks. The weapon must be repaired before being used again, which costs one-quarter of the weapon’s total price and requires a successful DC 15 Craft (Gun smithing) check.[/TD] [/TR] [TR] [TD]5-7[/TD] [TD]6-8[/TD] [TD]7-9[/TD] [TD]8-10[/TD] [TD]Something prevents the weapon’s firing mechanism from working. Determining and fixing the problem requires a full-round action. The firearm may not be used again, until the problem is fixed.[/TD] [/TR] [TR] [TD]8-10[/TD] [TD]9-11[/TD] [TD]10-12[/TD] [TD]11-13[/TD] [TD]Moisture or the wrong amount of gunpowder prevents the firearm from firing. The firearm must be reloaded before it can be fired again.[/TD] [/TR] [TR] [TD]11-14[/TD] [TD]12-15[/TD] [TD]13-16[/TD] [TD]14-17[/TD] [TD]A mechanical problem prevents the firearm from firing. The attack fails this round but may be retried with a new attack without reloading.[/TD] [/TR] [TR] [TD]15-16[/TD] [TD]16-17[/TD] [TD]17-19[/TD] [TD]18-19[/TD] [TD]An imperfection in the muzzle causes this shot to go astray. The firearm’s user makes another attack on another creature within 5 feet of the original target, randomly determining if there are multiple targets. If there are no other targets within 5 feet, the shot simply misses.[/TD] [/TR] [TR] [TD]17-20[/TD] [TD]18-20[/TD] [TD]20[/TD] [TD]20[/TD] [TD]No effect, the firearm misses as normal.[/TD] [/TR] [/TABLE] (Enhanced and masterwork weapons add their enhancement bonus to their roll on the misfire table.)
Some context to this table. We run a pirate campaign, and in our system Dueling Pistols are considered as masterwork weapons. They are the least prone to horrible failures. Firearms that have more than one barrel, or which fire multiple bullets at once, are more likely to result in horrible mishaps. And holdout pistols are perhaps the most dangerous (just like they were in real life). They are basically tiny firearms, designed to fire only one shot, while being easy to conceal. The downside is that they are just as likely to blow your own hand off, as they are to kill the person you aim it at. So our misfire table reflects that.
I'm also a huge fan of 3.5's Fortitude, Reflex and Wisdom save system. It just works, and I see no good reason why they had to change it for 5th edition. You don't need more than just those three saves.