[JRLSH]

I would like some information on reloading for .50BMG. Headspace! I have been reading about headspace and its importance and am somewhat confused. Headspace, as I understand it, is the distance from the middle of the shoulders of a case(or where the shoulders sit against the breech of the barrel in the chamber) to the base of said case. The shoulders sit on the chamber walls of the breech of the barrel. So when I full length size the cases I measure the length of my trimmed cases and all are within specs, 3.910, and the shoulders look and measure just as the specs in the manual say they should. My question is this: If I have shot a new round in my gun and have now fireformed the case to my guns breech do I still need to worry about headspace? Or is the headspace now set for my gun? These newly fired cases are going to be worked on a neck sizing die only and not on a full length sizing die so would headspace still be an issue? Thanks to all in advance.