I have tried a series of modifications to the factory muffler, and believe me when I tell you that a conservative approach is a complete waste of time and energy. Here's what to do:
- Unscrew the screws that hold the endcap on and remove the star-shaped aluminuim fastener that retains the screen in the tailpipe.
- The endcap is silicone glued on, so to remove it you will very likely need to insert a large diameter wooden dowel into the tailpipe for some leverage. You'll probably also need a rubber mallet or a block of wood and a regular hammer to tap on the edge of the endcap to get it off.
- With the endcap removed, you can see that there is an outer tailpipe and a much smaller diameter inner pipe. It's the inner pipe that is the enemy here. That thing is horribly restrictive, won't let the engine breathe, kills performance, and retains enough heat in the muffler to cook your thigh. It needs to go. Do not bother shortening it, rounding the ends, drilling holes in it, etc. All will produce tiny changes that will fail to satisfy you, and you'll be back inside the can again and again trying to figure out what more you can do. Forget it. I've been there. The inner pipe must be removed entirely to gain anything even remotely satisfying.
- You can see that there is a "step" where the inner and outer pipes are welded together. It's like a washer, really, with the outer diameter welded to the outer pipe, and the inner pipe welded to the hole in the middle. Get your handy-dandy hack saw and cut around the circumfrence of the outer pipe just behind that step/washer. This will remove the inner pipe but retain it intact in case some stupid regulation you might encounter in the future forces you to weld it back on.
- Honestly, this may not be enough, but put the endcap back on and ride it a couple of times. You'll notice the improvement in the way the motorcycle sounds for certain, and you will probably also notice an improvement in response to throttle inputs and less heat on your leg. But again, it might not be enough. If it isn't, you could try drilling a small hole (or several small holes, or a big damn hole) in the conical thing inside the muffler itself. I haven't done this and don't know how much of an improvement you may get by doing it, but it will certainly defeat any semblance of spark arrestor capability the factory muffler ever had.
- When you're through monkeying around with that monstrosity and want to see how the engine designers intended the thing to really perform, buy an FMF Factory 4.1 slip-on muffler.
I put a FMF 4.1 on my FS 570 and its a new bike.I also went with DNA filter and smog delete Hop on amazon or e bay spend 
