This is a DIRECT MIG gun replacement for the Millermatic 355. 

Industrial 300-Amp MIG gun with consumables for .035 in (0.9 mm) wire. 

Industrial-grade BTB semi-automatic air-cooled MIG guns are engineered to accommodate each welder's neck, handle and trigger preferences for increased weld access, comfort and productivity. Plus, these gun variations can all be supported with a common line of high-performing consumables - Featuring AccuLock™ S consumables - which simplifies ongoing maintenance and inventory costs. This unique combination of gun configurability and simplified support provides a total cost of ownership that's difficult to match.