My Senco 700XL Nail gun misfires a lot!

You can check a few things to identify the problem if your Senco FramePro 602 nail gun is not shooting at angles:

Make sure the depth adjustment is adjusted appropriately for the material you are dealing with and the nail size by checking it.
The cannon may not be able to fire at an angle if the depth adjustment is set too high.

Verify the air pressure: Ensure that the nail gun's air pressure is set properly.
For the FramePro 602, a pressure range of 70 to 120 PSI is advised.
The nail gun may stop working if the air pressure is too low.

Examine the magazine for any jammed nails that might be stopping the rifle from shooting properly. If you find any, remove them.
Make sure the magazine is free of any debris and unblock any nails that are stuck.

Verify the angle adjustment: Ensure that the nail gun's angle adjustment is correctly set for the angle you intend to utilize.
The FramePro 602 is capable of firing at 21-degree and 28-degree angles, so make sure that the angle adjustment is set to the appropriate level.

If your using refills that have wire holding the nails together.
Check to see if bits of wire, debris is collecting in around the striking hammer. I switched to paper and never looked back. Must have gone through 5k worth of nails and rarely jams. Other things to check,
1. Check the striking hammer, make sure it is not bent or damaged.
2. Check air pressure, Low pressure can cause jamming, misfires.
3. If the tool needs pneumatic oil, 2-3 drops before you start good rule of thumb.
4. Check spring return. Sufficient pressure must be maintained or the nails will not feed properly. Misfires, incorrect feed, jamming etc.
I had a Hitachi brad nailer and it was horrible. Fire 1 brad nail would jam or misfire. Did everything I could think of nothing worked. Put it up, got a Rigid brad nailer and never looked at the Hitachi again. :)
Sometimes you just get a lemon! (PS I understand you have a framing nail gun. Brad nail gun above is used for an example)
Hope this helps

One question is do you regularly put oil in the gun? Also you need to make sure that the screw holding the 'clip' near the barrel is tight and not loose. These are the 2 main problems when a gun is misfiring.

Sfn1 is an oilless nailer. It needs about 100psi for full power. If oil has been used in the gun or if oil is leaking out of compessor and making its way into the gun, the rubber firing valve could swell preventing it from working correctly. The firing valve sits on top of cylinder (black plastic). If the driver is bent or piston fits too tight in the cylinder you will have similar problem. The piston/driver should move easily down cylinder (but not slip down under its own weight). Finally, the nail will not be driven all the way in if the tip if the driver has broken off. Good luck with your repair.

First, check that you are using the proper nails. The FP602 needs Full Round Head nails with plastic collation. If you are using Clipped Head nails with the paper tape collation the angle of the nails not let the driver catch the next nail and drive it into your work.
If you have the correct nails, check the magazine to make sure the nails are feeding all the way up to the driver. If there is anything blocking the way or if the spring on the feeder isn't taught enough to pull the clip all the way up the driver won't be able to catch the next nail and drive it in.

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 need a top end rebuild kit

There is not an adjustment for this. You can buy an o-ring kit for this gun that comes with instructions and is easy to install. When the O-rings get worn or to much dirt and grit build up inside the piston and driver will not return all the way up after firing.

gun nails frist nail then does not set next nail

This is a discontinued tool by Senco but there are still parts available for it. It sound to me that the o rings and possibly the firing valve need to be replaced. I would recommend going to Senco.com and you will find a complete parts diagram in their discontinued tool section. They also have a toll free number if needed and you can talk to their air pro. I have rebuilt quite a few of these and they are not that hard just make sure to clean all the parts and use lube when replacing O rings.