Sig shipped my 4.5" Gen 1 a few weeks ago...about 3 weeks AFTER I took delivery of the Gen 1 3-Lug ILWT Barrel. I've shot all 3 barrels, 8", 4.5"(sig), and 4.5"(ILWT) and the ILWT barrel groups the WORST, out of the 3 barrels, in my Gen 1 MPX. As shown below...yeah..its a little longer with the HKParts 3-Lug Adapter, but its just as accurate as the factory 8" barrel at 25 yards...bench and off hand. I really wanted to like the ILWT barrel..cuz of the machined in 3-Lug mount..but I like the tighter group from the factory 4.5" Gen 1 Sig barrel.
Ohh...also, the ILWT had noticeably the worst recoil. The Gas Block and Tappet were from ILWT and I had to buy a gas valve from Sig...wondering if the ILTW has a smaller gas port than the factory sig barrels.
Here is a picture of all three for comparison.