Use a 1" pipe you can buy at Home Depot or Lowes
Insert to inlet side & hold muffler in upright position with both hands then hammer down to jar loose baffle.
Works every time, thanks to Steve's instruction.
That only works with the stock cores. A different approach is required to remove the Fullsac cores which are open all the way through.
I just removed mine to install the screens/sleeves and had to use a rubber mallet around the perimeter of the outlet end of the muffler.
I wrapped the outlet end of the core with a piece of bicycle inner tube to prevent it from being marred up and placed it into the jaws of a large channel lock plier. I slid the channel lock handles into a vise below the jaws and tightened it (my vice has a perfect area below the jaws to hold the handles that prevents them from moving while I am persuading the muffler can with my mallet. I then used a block of wood and a rubber mallet to persuade the muffler can to slide off the core. It took some love but they came out without damaging anything.
The inlet end of my core is pressed over a inlet pipe having a mesh type gasket attached to it. I think this is what made mine hard to come out. When I put them in originally and again this time, I had to persuade them in the last .25-.5 inched with a rubber mallet.
I though about drilling a vertical hole through the core just before the plate on the outlet end and installing a bolt across it before putting them back in. This would enable me to knock them out in the future using a dowel just like the OE core. Since I didn't know how this would impact performance or sound, and In was concerned that it may come loose and rattle I choose not to.