My makita 18 gauge nailer (af505) has started to misfire, it fires like it should, but no nail comes out. sometimes it will misfire once in every ten, and sometimes it misfires 10 in a row, there's no pattern to the misfire. its driving me mad, any ideas?

not enough pressure or the release is stuck

Your nailer uses 18 guage straight nails, from 5/8" to 2" long. As long as you meet these requirements it doesn't matter if they are basic bright, galvanized or staniless nails. If you Google or Bing search "18 guage stainless steel nails" you will find several mail order places that carry the nails you need. Not very many places carry stainless steel nails in stock because of their cost. Unless you live in an area where their use is predominant you will have to order 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.

I would try different brads for it, dont use anything generic, sometimes the glue that bonds the brads will jam up gun

first check for the proper guage nails, then look for a jammed nail in the magazine area and also a bent driver and lastly make sure that when fired the piston retracts fully all the above will cause the misfire.

Not sure about Senco but most have latch you can open to expose firing mechanism. Nail sometimes gets jammed and will not allow driver to fire or if not nail push driver ****** all the way back into gun then see if gun will fire.

you would need to check the driver/piston for problems and make sure that you are using the right guage nails. Let me know what you find and I will try and help further.

Try another brand of nails -- maybe your gun doesn't like those. Also, blow any dirt out of the nail magazine.

Take head off of tool. Clean and lube orings on the head valve . Do not use wd40 use air nailer oil.

is that a model AF505? if it is, your problem may be the gauge. make sure you use 18 gauge nails, he 16 gauge are too big and will jam in the gun.