[PlumberjimC]

How are SG barrel lengths measured? From muzzle to front of chamber or
bolt face?

[foghornl]

Unless I am way lost ....

Close action. Measure from bolt/receiver face to muzzle.

[Preacherman]

Yep... Close the action. Get a wooden dowel or rod from your local hardware store (note - using a long stick from the woods is NOT a good idea, as it can scratch the bore or leave dirt on the barrel wall that will be ground in there by the next passage of shot!). Drop the dowel down the muzzle until it contacts the bolt: mark the dowel level with the end of the muzzle: withdraw dowel and measure from end to mark. That's your barrel length.