Hrm. I think you have the definition of headspace wrong.
Headspace is measured from the bolt face to the inside of the barrel extention. Imagine if you will, where your .223/5.56 round (bullet) goes when you load up your AR. The very end of the barrel there is a cavity (chamber) where the bullet goes into. The chamber is loacted in the barrel extention, and is located inside the upper receiver. The bolt closes up the chamber, holding the round in place. Headspace is measured inside chamber.
Even if you had to remove the barrel when you changed out the handguards, you would not have changed the dimensions of your chamber. Headspacing is not required.
You want to measure headspacing, if you had shot a lot of rounds though your gun. The chamber tends to erode and get larger with higher round counts. If you have a lot of rounds in your gun, it is a good idea to headspace your rifle.
If you are buying a used rifle and do not know for sure the round count on it, its a good idea to headspace your gun.
If you just changed the reciever extention on your barrel (this is very unlikely as most people would just buy another complete barrel) headspacing should be done.
If you are looking to make match grade hand loads and want to maximize OAL then headspacing should be done.
Assumption is the mother of all ****ups.
Know what you know, Know what you don't know. -Paul Pedzolt
"Jack of all trades, master of none, though oftentimes better than master of one" -Mynshul
Last edited by AgentTikki; 12-11-2012 at 08:44 PM.