Grover and Foxtrot this is the email I got from Windham about the chamber on the varmint exterminator.

Hi Robert,
The chambers on the stainless varmint guns have what is called a 5.56 Compass Lake chamber with a bolt matched to the chamber for match grade headspace. The leade in this chamber is somewhere between .223 and 5.56 NATO. Not as tight as .223 but a bit tighter than the NATO spec.
These are also marked .223 rather than 5.56. We do this primarily because the chamber is a little tighter and better suited to match grade .223 ammo. While the chamber is tighter than 5.56 in the VEX, the leade is still longer than .223 Remington. We have tested these rifles with 5.56 ammo as well and they are perfectly safe with that ammo too.

Cheryl Eliason
Customer Service Manager
Office: 207-893-2223

Also the Windham has a life time warrante that is transferable to a new owner if you sell it.
If you go to compass lake engineering you will see they offer the compass lake chamber in kreiger and douglas barrels, which are both supposed to be top of the line barrels. The compass lake chamber is nothing new and was not created by Windham but by compass lake which is a barrel specialist. They also sell uppers and lowers, complete rifles, triggers and other accessories.
My hand loads are all 223 in LC brass, I have never seen any 5.56 hand loading data until the new Hornady book came out recently. There are no available dies for resizing 5.56 brass, 5.56 brass that is resized is done in a 223 sizing die. Thus the only difference is the thickness of the brass case. Even the Colt's and other rifles are chambered for shooting both 5.56 and 223 ammo.