That's completely normal for a Parker up to the late part of the century and on demand or to use up stock barrels after that. At that period it should have a normal paper shell chamber. The same was true of the 10 gauge guns which had 9 gauge barrels in the early years and were also available with paper or brass chambers. One Parker I own is a first model 12 gauge with back action locks. It has 11 gauge barrels by measurement and was apparently sent back to the factory just before WWII to have the chambers enlarged to "standard 12 gauge chambers". I found it interesting that the work was the only documented record for the gun. The gun was made in 1869-1872 with a a5xx serial number. It says a lot about the gun that over 40 years after the original purchase, the owner got tired of having to handload obselete undersized brass shells and sent it back to the factory for the $1.50 job of rechambering. He must of really liked the gun.
