Marty, Sounds like you got a newer muffler simular to the one on my '02. I did the Hartman mod last year and found the same in my muffler. All I ended up doing was  cut the spotwelds at either end of the rounded outside chamber and slip(hammer) it off the inside pipe. the inside pipe has a bunch of holes drilled in it where the outside chamber sits. I didn't drill out any of the holes in the inside pipe but it might be a good idea if you want to really increase the exhaust note. Anyways I packed the rear chamber with fiberglass packing, slipped the pipe back into the center baffle and welded the end back together. I was happy with the results and only changed to a sportster muffler because I was able to get one for free. If you repack the rear chamber with fiberglass don't start the bike in a garage, DO IT OUTSIDE!!! the fiberglass goes everywhere, up your nose, in you hair, on you face, in your shorts, everywhere you can think of and some places you didn't know you had. It's a good modification and well worth the time. You also will notice a increase in the bike's power as soon as you open up the stock muffler.Hope this helps. Ride Safe

Lot of questions, the muffler mod moved what I call the comfortable shift point up by about 400 rpms from around 4200rpms to 4600rpms. I then changed out the white spacer with 2 #4 washers and noticed another increase in rpms, maybe another 200rpms but had a lot more torque midrange. I didn't get much riding in with the bike in that setup so I couldn't really check it out properly. I did the rejet to a 152.5 and the sportster muffler at the same time so I'm not sure which mod gave the best results. With you muffler try drilling out eight to twelve of the holes with a 3/8 bit, that should give you plenty of flow The more you drill out the louder it will get. I used some fiberglass repacking cloth that I got from J.C. Whitney but any bike shop should have it or get it for you. The rejetting will be the biggest power increase and with the modified stock exhaust you should be able to use either a 150 or 152.5 main. To play it safe I would start with a 150 and if the bike runs right then think about the 152.5. After I rejetted the engine runs up to 5600rpms with ease, almost got into some trouble getting into the turns a bit hotter than before. You will find that you are only in second gear where you used to be working third.
 I didn't redrill any of the holes in the inner pipe of the muffler, looking back on it I wish I had. The Sportster muffler is only a little bit louder than the modified stock one which is why I think drilling out the holes is the way to go. I only changed to the sportster muffler cause I was able to trade for it and the stock muffler was getting a bit banged up from a couple of low speed crashes. The only other tip I have would to be remove the door over the airfilter to help the airflow. I'm thinking about taking mine off, drilling it out using a hole saw and the covering it with screen to keep the critters and leaves out.
 Oh yeah, I'm using a self contained engine tach that I got from Northern Hydralics, it is made for small engines and all you do is wrap the pickup wire around the plug lead and hook up a ground. I mounted it between the risers on the handle bar with ty wraps. So far it has been well worth the 30 dollars so you can see the difference the mods make. Have fun with it, that's what it is all about.  , Ride Safe