The gun will fire staples, nails do not set
The gun will fire staples, but when you go to fire a nail or brad it will not set the nail, it bends the end of the nail.
Air Tools & Compressors - Stanley - Bostitch Sb 2IN1 2 In 1 Narrow Crown Stapler/18 Gauge Brad...
Answers & Comments
Shoots staples fine but brads jam or bend. Check that the part called "guide bar", #35 on the parts diagram isn't getting stuck in the channel of part #30.
The guide bar retracts into the channel when driving a staple and springs out of the channel to guide brads.
The guide bar can get stuck in the channel if it's bent or gummed up. (Bent is likely the result of driving a brad into an obstruction.)
The business end should look like this: