Here's the 357AR (Max rimless) I built for my son last year. It's a decent little round that's on par with 35rem (at standard pressures) with similar barrel lengths. Built the first upper in it in 2012 and worked out the bugs. I carried a pistol chambered in it for the 2013 deer season and of course saw no deer. State land, saw lots of other guys though. 2014 MI allowed pistol caliber rifles so I was all set and got a 6pt in the same spot (had 13 guys wander by the stand first). 2015 we got a peaceful private land spot and my son was finally able to gun hunt. Figured it was time we made him one. I had previously made a few chamber reamers and set of headspace gauges. While the cartridge is basically the same as a 357Max, you need to have a headspace gauge that sets the headspace off of the case mouth. We ordered a .358"/.350" Green Mountain 1:14" barrel blank http://www.gmriflebarrel.com/rx358-3...-barrel-blank/ . I already had several barrel extensions, an upper, extra bolt carrier group, and an unfinished 80% lower. The barrel was set up with a pin gauge and beveled so the barrel section could be turned between centers down to 1". Once that was done the chamber end was cut for the barrel extension threads 13/16"-16tpi .620"(ish) long. The bolt was put in the barrel extension in the locked position and the distance from the front of the bolt to the front of the barrel extension. Subtract .010" for clearance and Around .004" for crush. Also measure the bolt face depth and add at least .020"-.050" or more to get your rough Go-gauge protrusion for headspacing and write that number down. After verifying the barrel extension screws on smoothly the barrel was reamed with the reamer held in a floating reamer holder. I mark the reamer with a sharpie for depth and once you're close to that, clean the chamber, threads, extension and bolt. Insert the "go-gauge" and with the bolt locked into the extension, lightly screw on the barrel extension until you feel resistance. Use a feeler gauge to measure the gap between the barrel shoulder and extension. I generally loose .003" to .005" clearance once the extension is torqued down. In the pic the (ar10 best pic I had of it) barrel extension is .003" from closing on the "go-gauge". I cut the chamber .006" deeper knowing that I'd most likely have to touch it up a thousand or two once the extension was torqued down. Turns out it was spot on. You can feel a slight bit of resistance with the "go-gauge" and the "no-go" won't even begin to turn.