It's a small shank. Large shanks actually have a shoulder right in front of the barrel nut, so you can see the threaded portion is larger than normal.
Check with Jim at Apache Gun Works. He's a site sponser, does GREAT work, and has a few different options now as far as barrel blanks go, depending on what you want to spend. And he's getting ready to open his ordering back up. He cuts it off when he gets a certain number of orders, to try to keep his lead time around a month.
http://www.Apachegunworks.com
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