The PW in the Circle shown in your picture is a Winchester Proof Mark.
The purpose of a "Proof Mark" is to indicate that the part in question, in this case your barrel, has been tested with a Proof Load and is safe to fire with the designated ammunition and loads for the weapon.
The Constitution is not an instrument for the Government to restrain the people. It is an instrument for the people to restrain the Government!
Today, we need a nation of Minutemen, citizens who are not only prepared to take arms, but citizens who regard the preservation of freedom as the basic purpose of their daily life and who are willing to consciously work and sacrifice for that freedom."
-- John F. Kennedy