late 2011 after hunting season I had gathered enough parts for a build I wanted to try out. An AR15 in 357Max only without the rim. I had a cheap $30 357/38 barrel blank, barrel extension, spare upper, BCG and enough extra parts to finish up an upper. Didn't want to pay for a custom reamer to test it out in case it didn't work out. A standard 357Max reamer cuts a leade starting at the case mouth so you can also shoot shorter 38 and 357 ammo. Since this would be headspacing off of the case mouth I needed a reamer that would cut a case mouth step. After making a few dummy rounds a reamer print was made and from the reamer print I machined a reamer on my lathe and mill. Standard 357Max head space gauges will not work. Being a straight walled case the headspace gauges are easy to make as they can be as simple as a cylinder of a certain length, but an extractor groove makes them more usable. Cut the chamber and installed the barrel extension verifying headspace. The feed ramps were opened up with a 3/8" carbide ball end mill, but I've since gone to a modified center feed type. The magazines need the bullet guides removed from the front of the mag so the cases can double stack which is easy to do on Pmags using a dremel and a sanding drum. The test barrel went well and cycled several different bullet types so the test barrel was sectioned and a new GMB .358" 1:14" blank was purchased for a better build.