You don't need to get a new gun - just stack a few small washers on the tail of the rivet - just enough to get clearance for the nose of the gun. Too many though, and there won't be enought "tail" for the gun to grab onto.
I've even used a small nut before. It's not perfect, but it works.
Since you're doing the whole track, I would consider modifying the nose of the gun to fit the track. I used a Dremel tool to cut away enough material so that the nose fits into the track.
Two tricks for installing the track:
- Use a large drill bit to slightly (and I mean slightly
) increase the countersinks on the track. It's OK for the rivet heads to be below the bottom of the track. It's very bad for them to be above it - they'll catch the bottom of the traveller, which can hang up on one side when you least want it to hang up.
- Use duct tape to hold the track down and in place while you rivet it. The track has to be firmly seated on the crossbar and the rivet firmly seated in the hole. The rivet will not pull the track down if it's loose, and you end up with loose rivets.
Good luck! (I hope you bought a good gun - you've got a lot of rivets to pull!)