What is checking the headspace in upper?
Typically this means checking for excessive headspace which would be dangerous. If the bolt won’t close on a USGI FIELD gauge, then headspace is called ‘good’.
I like to have my guns not close on a NO-GO gauge nor a FIELD but that always doesn’t happen on a 5.56mm NATO chambered gun unless I have several (like a dozen or more) bolts to try.
I have a LMT upper and LMT BCG is this something I need to have checked?
If both parts are new no. If you had an old bolt or old barrel it would be a good idea to check.
thanks!