along wrote:"the go gauge closes with when rollers extended are 4mm away from the roller slot end".
Uh, no. If the rollers are 4mm away from the ends of the roller slots, it didn't "close".
Headspace is measured when the rollers are FULLY OUTWARD, at the ends of their slots. If the rollers aren't fully outward, the bolt isn't closed, it's ALMOST closed. Measuring how far the rollers are away from the ends of the slots just gives you an abstract idea of how much too tight the headspace is.
If the rollers won't go fully outward on the go gauge, headspace is too tight. The bolt isn't considered "closed" until the rollers are fully outward at the ends of the slots. The equivalent for a rifle would be not having the bolt handle able to turn completely down with a gauge in the chamber. The rifle isn't ready to fire unless the bolt handle is fully down. Same for the MG42, only it has rollers in slots, instead of lugs in the receiver grooves. The principles and methods are the same, really.
You wouldn't fire a rifle with the bolt fully locked (handle fully down), and you don't want to have the MG42 fire with the bolt not fully locked (rollers FULLY outward). It is embarrassing to have someone picking brass shrapnel out of your arm.
I suggest you read (or re-read) the above messages, and the link below:viewtopic.php?f=39&t=4907&p=91369&hilit=headspace#p91369