Normally I'm a big advocate of NOT re-inventing the wheel, and folding existing rules into new mechanics.
However, the problem I'm seeing with the a gun jam causing the Broken condition, is that it's not just a simple -2 to attack roll (and reduction of crit range). The Broken condition interacts with a number of other rules mechanics that the gun's misfires doesn't really take into account.
The Issues
1. The Broken condition normally occurs when the object is at less than half hitpoints. This is important, because it denotes when it no longer has the broken condition when being repaired.
For spells or abilities or craft checks that repair normal broken items, how does this interact with misfires?
Do misfires automatically bring the weapon down to half hitpoints - 1? If they don't (and it's not mentioned anywhere), then how do we know the item's repaired when we, say, cast mend or make whole?
2. The misfire is obviously supposed to indicate either a jam in the gun, or a dirty barrel causing poor performance, or some other similar effect. A non-gunslinger, using the craft skill, would need 1 hour of work (and a DC 20 check) per point of damage repaired. First, cleaning a gun out going from 1 hour minimum to "quick clear" speeds breaks my immersion.
Second, taking a week to clean the barrel of a gun from a single misfire if misfires reduce hitpoints to below half sounds like insanity.
3. This ties in a bit with #1, but what if someone wants to Sunder the gun? Perfectly reasonable response.. it's an expensive item and it's scary and dangerous.
If the gun gains the broken condition from being sundered, does it now have a higher misfire chance and risk blowing up?
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Proposed Solution
All these things really point me towards desiring a new, "middle ground" level of brokenness for a "first stage" misfire.
I'd call it something like "Compromised". This item state can grant the misfire increase to guns, and maybe a -2 to attack for weapons (doubles armor check penalty on armor, -1 penalty on tools, 50% chance of using an extra charge on wands/staves, etc).
More importantly, it doesn't cause hitpoint damage. To fix it, it takes one application of a spell (depending on if it's magical), or one craft check that takes a specific set amount of time (1 round action, or 1 minute if we decide/prefer to make it non-combat allowed).
This feels more appropriate to a simple "gun jam" or "dirty barrel". It also opens the door for applying this kind of condition to other items (0th or 1st level spells that cause it, or conditions to apply in certain environments, etc).
A gun misfiring while already jammed would have the same effect as before (destroyed in a shrapnel explosion). If some DMs want to be more lenient, instead of destroyed after the explosion, then we can have the gun sit at a broken status with 1 hitpoint (similar to what can be allowed with a sunder), but a special caveat of "unusable" until repaired.
A blown up gun taking a long time to repair still makes sense.