2 nails shooting at once from nail gun

The manual for your gun specifies 34-degree nails from 2 to 3 1/2 inches long. 28-degree, wire collated nails are a Stanley-Bostitch thing and they only work in their nail guns, they will not work in your nail gun. You need 34-degree, paper-tape collated nails. I know Senco framing nails will work in your gun and I think Duo-fast makes 34-drgree nails but check that before buying them.

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.

Check that the nail is correct for the gun. Fifteen gauge is a very stout/strong nail and measures around .060. Place a strip of nails along the outside of the rails with nails aligned with rails. The first nail should be aligned with path of driver. If the angle is off do not use. Look at the thickness of the driver (and or condition of, no damage ), the nail should look about the same thickness. If nail is thinner than driver, the second nail in line will be in the path of driver. This could cause damage to gun. I believe that the nails slide in from rear.

Most nail guns are labeled with the longest nail/brad they can shoot but are capable of shooting shorter nails/brads. There should be a data sticker on the gun, usually on the magazine rail, or at least in the manual that tells you what pressure to operate the gun at, what guage brads the gun can shoot and the size of brads from shortest to longest it can handle.

This is iffy... but PC nailers are nit picky. If you used someone elses nails, it can (and will) jam and mess up.
So, if Porter Cable nails are being used, clean and lube the nail shoot (with a few drops of air tool oil).

If that does not work, Invest 19 dollars in a new, 18 guage nailer at Harbor Freight. I have 2 of those and they have been trouble free for 3 years.

Yes. Roofing nailers all use about the same size and spacing on coil nails. You will find that Harbor Freight is about 1/2 the price of Home Depot or Lowes for those nails.
-Jim

yes ,should be on head of gun,if not try turning air preasure up.

I worked at at tool rental center while attending college and learned from the old tool masters about nails and nailers. Rental stores have many different brands of nailers to rent and customers would bring their own nails of all brands. You will notice on the case/box that your nails will /should work on many other brands however the degree or angle of the nails is not always exacly as stated on the box. You should check with protractor the exact angle of your nails and also check the exact angle of the nailer. Use straight edge on travel path of the piston and another on guide rails to check angle of the gun. You will discover that all is not as stated as in a perfect world. If the angle is off slightly, the head of the second nail will get into the path of the driver. This happens mostly with clip head because the nail heads are so close together. The full round head is spaced further apart with less jamming. If angle is off slightly, take a look at other brands of nails with protractor in hand, the angle will be different slightly or choose a slightly thicker nail. Look for the nails to fit with bottom of nail hitting the barrel first. This will move the head of the nail slightly away from the center line. Finally do not release pusher with force as this will jamb nails closer together. Good Luck!?

The dial on the top of the gun is most likely the depth adjustment for the nails, this should solve the driver damaging the work piece. Make sure the nails are the correct guage for the nailer you have.

This happens to my gun too. You probably need to oil the gun once in a while. Here is how you fix it:
Disconnect the gun from the compressor first don't wanna get hurt or anything like that. There is a pin that shoots the nail out well that pin is jammed so what you have to do is push that pin to the original position. using a flat head screwdriver would help you to push that pin back into place. Let me know how that turns out please. I pretty sure that is the problem with your gun. Make sure you lubricate the Nailer once in a while.