No damage to you, your weapon or the rest of the muffler. I would call that a win. Nice work, just some small modifications left to do. It’s been years since I was in the machine shop but I believe there was a 417 ? stainless, maybe had better tensile strength.
I checked the other metal shop down the road. They had a small pile of stuff in a back room. There was stellite, titanium, inconel, etc… The only 17-4 they had was too small. I grabbed a piece of nitronic 60. It is supposed to be almost twice as strong as 304, so it should work fine.