It's similar to a Wylde chamber; crossbreed.
I wouldn't limit yourself to a Windham build; there are tons of manufacturers out there and varmint rigs aren't as popular so you shouldn't have to compete with all the panic buyers currently looking for a M4forgery.
I bought my varmint upper from Fulton Armory. I went with a 24" barrel to harvest the extra velocity. Their house barrels are made by Krieger.
Hk USP 45 Expert, USP 40, USP 9, SIG P226, S&W M&P9, Hk 4, Makarov, Desert Eagle 40, Beretta FS92 Centurion, Kimber TLEII 1911A1, Glock 22, SIG P225, 1943 Rem Rand 1911A1, S&W mod 64, Walther PPQ, SIG P229, Browning BDA 45 (SIG P220), Hk45, SIG P230