I know the threads on a barrel are 1/2 28. But i don't know if the 1/2 milled part is 3/4 back from end or if the threads 3/4 long?
                                                                            Thanks Tony

Per drawing 9349054, the distance from the very forward end of the barrel back to the shoulder is 0.62 +/- 0.02.  

The drawing also calls out an area just forward of the shoulder that is 0.025+0.014 deep (below the 0.500 OD threaded area) and extends forward ".100 max to first full thd".  

The specified radius transitioning from the face of the shoulder into this reduced OD area behind the last full thread is 0.021-0.010 R.

The specified thread for the Flash Supressor / Muzzle device is 0.5000-28UNF-2A

thanks for the help
                            TONY