Here is what I've been working on this past winter with a lot of imput with several members over at weaponsguild. An ar15 based off of a 357 maximum rimless loaded to 223 pressures. I started with a home made chamber reamer (because standard 357max reamers won't work) and a 357 test barrel and once I got the bugs worked out I sectioned the test barrel and bought a Green Mountain blank to turn down. I haven't run it over a chronograph yet, but I was able to go above what Quickload shows for a 158gr bullet @ 2250fps without any signs of high pressure. I'm currently working up loads for 180gr bullets. Here's a few pics.