Okay, you can use your scatter attack (cone) to shoot 2d6 fire (Reflex for 1/2).
Drawback is you count as misfiring if either (and both) rolls are 1's.
This seems you are doubling misfire chance.
A normal Blunderbuss is Misfire Nat 1-2. So 2 times out of a d20 (2 outcomes).
The chance to roll a 1 on a 2d6 is 3 outcomes, when one turns out a 1, when the other does, and when both do.
Why did pathfinder think it was worth it for a tiny boost in damage (elemental at that)?
Compare a dragon's breath alchemical cartridge with alchemist's fire:
2d6 damage vs. 1d6 damage plus 1d6 damage next round (1 damage splash)
15 ft cone vs. single target (5 ft radius splash)
40 gp vs. 20 gp
negligible weight (~60 per lb) vs. 1 lb
Even compared to a wand of burning hands (CL 2) (market price 30 gp per charge, save DC 11), the dragon's breath cartridge isn't bad. It has a higher save DC (15), can be used by anyone (no spell list requirement or Use Magic Device checks), "charges" can be made/purchased at the desired quantity (instead of 50 at a time or GM-fiat), "charges" can be replenished easily (instead of requiring batches of 50), and the effect is non-magical (which can be important in certain circumstances).
The chance of misfire is a concern. The normal misfire is 2-in-20 (10%), while the dragon's breath misfire is 11-in-36 (slightly over 30%). However, the damage is nearly doubled (actually doubled if using a dragon pistol) and does elemental damage (bypassing DR).
All in all, it's an option that has some pros and some cons. It can be useful in some circumstances (better than the equivalents in several aspects), but has an increased chance of failure. IMO, the reward tends to outweigh the risk.
If you take Dragonchess Player's numbers a little further, you'll misfire 37.5% of the time (7 or 8 times out of every 20.)
It looks like if you have Grit to spare, it just takes a move action to fix the gun and can carry on with the fight just giving up your full-attack.
Maybe if you have more than one gun, you just drop the jammed one and keep fighting with your spare?
I guess in your original post, Starbuck_II, you were saying that you were concerned that it doubles your chances of misfire, but it looks like it nearly quadruples your chances. I guess if you're really counting on your character not misfiring you shouldn't use the cartridge.
It seems worth it to me in certain situations if you feel you can spare the Grit to fix the gun, or spare the Standard action to fix it if you can't spare the Grit, or you can spare the gun because you have extras.
If you take Dragonchess Player's numbers a little further, you'll misfire 37.5% of the time (7 or 8 times out of every 20.)
No, you misfire 11 out of 36 times(30.555555...%). Ultimate Combat pg. 140: "Because this ammunition forces a saving throw instead of making an attack roll, the misfire rules are slightly different. If you roll a 1 with either of the damage dice, the firearm misfires."
If you're really concerned with misfires, firearms are not for you until you can afford a 10,000 gp figurine of wondrous power (slate spider) or hit 11th level and take Signature Deed.
No, you misfire 11 out of 36 times(30.555555...%). Ultimate Combat pg. 140: "Because this ammunition forces a saving throw instead of making an attack roll, the misfire rules are slightly different. If you roll a 1 with either of the damage dice, the firearm misfires."
Ah, I see. I was including an attack roll misfire with the damage misfire.