William, unless this gun was special ordered from Marlin for 3" chambers, then it would have a stamp on the barrels water table. Ir would then have the standard 2 3/4" chambers, and I have seen some that have the 2 3/4" chambers on the same flat as the serial number. If it was opened by someone, I would still not shoot 3" loads from it. The guns at this time from Marlin were proofed at higher than nitro loads available at the time, and would be safe with the heavier loads offered THEN. This would not apply to today's modern Magnum 2 3/4" loads.
RWTF, If your Longrange is not stamped 3", then it is 2 3/4", unless someone opened it up. Not all Longrange or Wildfowl guns were 3" chambers.
My Longrange 3" chambered gun, the chamber gage goes to 2 15/16", and my gages are built from Hunter Arms Co. prints.
Last edited by JDW; 12/14/12 12:50 PM.
