I don't have an extra A2 type flash hider.
I just put my calipers on the bare threads and it is showing .49" so I guess that's 1/2x.
The only thing I can figure is this barrel was cut down or perhaps Wilson made barrels with 1/2x threads before 5/8x.
I know I can get a 1/2-28 to 5/8-24 adapter or cut the barrel down. I hate to go with an adapter because, well, adapters are open to issues although I would rocksett it. I hate to cut it down as it has a real nice handguard on it that fits just right.
I'll probably stop by Silencer Shop tomorrow and talk with Capitol Armory and see what they think. I'm thinking the adapter route would be fine, just hate to have an adapter but I know guys do that.
Other option would be selling the barrel and buying the same thing, threaded "right".