When i pull the trigger it rapid shoots nails mostly a quick two

Most likely stuck trigger valve. Easy to check. Remove nails, remove trigger to expose the firing pin of trigger valve. Connect air and very quickly depress the firing pin all the way in. If the gun fires , adjust the safety to depress the trigger valve completely. If the gun does not fire, replace firing valve, or remove and check /lube o-rings in valve. Good luck

if you mean it only shoots air when you pull trigger and not a nail , disconect from air supply and open side of nail mag. (two screws on side) look for forign object in front towards head end , sometimes the paper that nails are wrapped in gets stuck in there , also there is a small spring metal clip at front of mag is this broken off and is unusually short ?

Most nailers have a two position setting on the trigger to allow for single fire or bump fire. If set on bump fire, the gun will work better when you bump fire the gun instead of placing the gun on the work then depress trigger. Bump fire works best when safety spring has good tension. Easy test. Make sure that the safety spring is in place (with air hose removed, contact foot should spring back quickly when pushed against work). With air connected, hold trigger and quickly bounce the gun against the work. If the gun does not fire when you bump, the trigger is most likely set to single fire. With trigger set to single fire, you must place the gun against the work, then depress trigger. If the gun shoots two nails while set to single fire, most likely worn trigger or worn out trigger valve. These parts are not expensive and are easy to replace. Good luck

Check the safety trip make sure it is pushing back far enough. If it doesn't then it won't do anything. When it was fine just moments earlier.

Best practice is to bump fire the gun to prevent double fire. Do not place the gun on the work and pull the trigger. This takes practice while using at an angle. Works better if the nose piece has rather sharp points to help nose to stay in postion while you bump. With trigger depressed, practice without nails bumping/hitting wood at different angles untill you can bump hit at the spot that you need the fastener. If the gun double fires while bump-firing, most likely all the safety/trip parts are worn/loose. Not expensive to replace. Good luck.

Nailers that use strips of nails (one nail directly behind the other) will sometimes drive two nails at once and is caused by damage to the driver. The nails are so close together that the driver will hit the first and part of the second nail head. Coil nailers, as your an451, use nails seperated by welded wire making it very difficult the the driver to hit the second nail. Usually when a coil nailer shoots two nails, it is acutally the trigger valve that acutates twice (two very rapid fires). Two very rapid fires is caused by faulty trigger valve. I have seen a roofing coil nailer fire one nail and a second nail drops out of the gun. Caused by weak / malfunction of nail feeder or wire used to weld nails together is too thick.

Worn trigger and or worn firing valve. Remove the trigger and look for wear on contact surfaces (worn pin that holds trigger, worn inner lever inside trigger, worn or loose firing pin in valve ). Easy test for worn trigger. Remove the nails, remove trigger, fire the gun against wood with finger to test for proper action. If it works better without trigger, consider replacing trigger with new (I would also order the oring repair kit that includes the firing valve )

Bostitch nailers have two triggers available. Black trigger is rapid fire while the gray is single shot. Both triggers are usefull depending on type of work. Most people say that there is a degree of accuracy with single shot allowing for better placement of nail. If speed is your goal then use the black trigger to bump fire. More information and parts can be found at www.bostitch.com , look for "Support" tab, then "service / parts " find your model number from the pull-down list and search for the manual and parts list for needs. Good luck on your projects.

the 601 had a design flaw, in the nose, where the nails hit as the are being pushed forward, the peice was made out of a peice of metal that was too soft, it wore down and that is where you get the double nail problem.

Senco offered a trade in, this was years ago, for a limited time and gave us credit towards new guns, which we gladly did

I hope this doesn't offend you. What I'm suggesting is that you may not be using your nail gun correctly. Some people keep the trigger pulled and bounce the gun onto the wood. This technique works okay, but placement accuracy of the nail suffers. The other technique is to place the gun on the wood and pull the trigger USING TWO HANDS! This is the technique suggested by the manufacturer. The problem with this technique is that if you apply insufficient pressure to the gun (USING ONE HAND) the safety is not engage so you don't shoot a nail. What usually happens - I just witnessed this a couple of weeks ago - is that not enough pressure is applied while pulling the trigger so you push harder and end up bumping the gun which shoots a nail - because you have the trigger pulled, but the nail shot was a surprise so it causes another bump (and is more apt to bounce because you're using just one hand). Because it is a machine and can operate faster than your reflexes - it shoots another nail. This happens before you can lift it off of the wood, or remove your finger from the trigger. That's the reason I keep the trigger pulled and bounce it. I know that after the bounce I've set a nail, and can remove my finger from the trigger. When I need to accurately set a nail I always use two hands, press hard and pull the trigger, and release quickly. Only pull the trigger when you know you're about to set a nail, and are poised above the workpiece.