Minimum Barrel Diameter for Threading?

Looking to get my son’s newly acquired .22 threaded 1/2x28 and was curious on the minimum OD required to accomplish this. Any machinist type folks on here in the know?

Well 1/2 is the major diameter of the threads so you need at least that, preferably a little more - say 0.550. I found this thread guide from AAC that looks like it has some pertinent information.
http://www.silencertalk.com/AAC-thread-guide.pdf

Great. Thanks, Eric!