I rebuilt my NR93AC3 because the old body cracked. The gun first fired fine, then second shot hissed a bit before firing, third shot hissed a little longer before firing and on to the 12th shot. After the 12th shot the gun will only hiss and will not fire.

The barrel selector sounds like it is not engaging correctly to the switch over of trigger to firing pin for the second shot if you don't get it repaired it may get worse and go off when unexpected

Contact a local gun lovers shop.

Check the firing cap seals,,sometimes they crack,,next check the o rings that seal the cylinder to nailer body

I have seen this on dewalt and p/c guns. Usually caused by trigger valve but you mention that trigger valve is new. If the headvalve does not seat correctly on the top of the cylinder, the in-rush of air will leak past cylinder seal and push down the piston (feels like you fired the gun ) . Difficult to pin down the problem but could be headvalve is sticking slightly open. I would suspect headvalve orings too tight or spring in head is weak or cylinder in head is worn. Easy to check. Remove the head and push in the headvalve with thumbs. The valve should be stiff to depress but return freely. Check that the cylinder in the head where the outer headvalve oring makes contact is not scratched or worn. If scratched, sand with fine sandpaper till smooth. If worn and depressed, replace head. If spring feels weak, stretch it a bit pulling it apart. Lube cylinder /orings with white grease. Untill you find the problem, do not load nails untill after airline is connected. On the dewalt guns, firing when air is connected is due to the trigger firing pin orings worn. You could check this on your gun by spraying soapy water on the trigger valve under the trigger then connect the airline. If air is blowing out the trigger when you connect air, most likely faulty valve. Good luck and let me know what you find out on the unusual problem.

Most all of my guns are senco and the head has the moving part for fire and reload,, controlling the driver piston. Multiple shots suggests a trigger problem. No water in system right.. Well Its all O rings little ones,, skinny ones,, fat ones etc. You can open it up and check, do the head first, I think.. Its fast a off and on

Upon cocking your gun is there the same tension with the cocking spring as there was befor? Also check the condition of your barrel seal for tears or cracks in it.

Your o-rings are wore out. Paslode makes(made) a Tool Repair Kit, PN219235 and Trigger Valve Kit, PN219224 for your gun. Always use all of the o-rings in the kit and liberally lubricate them before installing them using grease in the kit or nail gun oil. If the kits are no longer available you'll have to get a breakdown of the gun, (www.paslode.com/uploadedFiles/PowerMaster_Plus_Parts_Diagram_Schematics.pdf) and order all of the o-rings. Be very carefull to put everythng back together properly, you are working with a gun.

They all leak a little on firing. Is it a big blast of air or just a hiss? All guns have a 'safety' device that keeps them from firing unless you follow a set pattern. You must push the gun down where you intend to put the staple which releases the safety before you pulll the trigger. You must then release the trigger move the stapler off the piece push it back down again before you can fire another staple. You can get (free) frrom the manufacture a rapid fire shoe and trigger that allows you to hold down the trigger and bounce the stapler along your work shooting merrily away.

It sounds like you've either have a weak firing pin spring, or the follower may be giving you problems. Did the gunsmith check for excessive headspace??? If not, he or she probably should. This can not only cause mis-fires, it may also create a dangerous situation otherwise. I'd DEFINITELY get a second gunsmith to check this out for your own safety! I'm a gunsmith myself, and wouldn't risk it ever.

There is one main part that can be the cause of the problem.

The trigger. Check to see if there is anything out of the ordinary with this. There should be some moving components attached to the trigger itself. Is any part of the trigger touching the button in the body of the gun when safety is not depressed. If the lever on the trigger is pushing on the button, even a little bit, It will allow the gun to fire again and again.