I am pretty sure it is a large shank. It is a custom build on a Savage Target Action and on Savages Part Finder it is listed this way 'TGT ACT 223 RIGHT PORT AT SS. I looked up the barrel nut and its size is listed as 1 1/8". I measured the length of the nut and it is 5/8". There is no step down in front of the nut and the barrel diameter is pretty consistent from shank to muzzle at 1.118" I would call the gunsmith that built the rifle but I want to order a pre-fit barrel and I feel he might not like me not going through him. I would rather not take the barrel off at this time to measure the thread diameter, as it is a wildcat and I don't want to go through head spacing it with a fired case.