Mine didn't fit either. Note the seem in the middle where I had to cut/weld it to make the angles right:
I also had to make my own bracket since nothing lined up correctly. This could be because my Riot was pre-production unit though:
Also note, here's my first of 2 blown out Big Gun mufflers. This was after 2.5 hours on the dunes. Packing completely gone, end cap blew off and the furthest point from camp and by the time I got back, it looked like this:
The second muffler lasted 4 hours and that was with me tightening the screws at every stop. After 4 hours the can was completely empty of packing and my head was ringing with ear plugs and a helmet on.
Here's a picture after the first stop that day with the second muffler. The packing was making its way out of the seem between the end cap and can. Note the Big Gun sticker is already turning brown due to lack of packing and also the end cap screws are working their way out.
Moral of the story is: If the muffler doesn't have upgraded packing, get it and install it. Closely watch the muffler the first few hours out...
As info, here's some pics of the Gibson installed. Baffled muffler with no packing to blow out. I ran it for a year without issue: