Wooden dowel
Best left to a gunsmith
While hunting, yes. Otherwise, no.
A shotgun typically stores 5 rounds in its magazine. In some countries, when hunting certain types of game, you are only allowed 3 rounds in your magazine. The "plug" therfore goes into the magazine and takes up the space that two rounds would occupy., reducing the shotgun's magazine capacity be 2 and making it legal. Thus removing the plug restores the shotgun's capacity for a full magazine.
If you are referring to the magazine tube plug. The answer would be No. It is put there for bird hunting regulations only.
Gun is restricted to 3 rounds by design. It has no mag plug, that is where the gas system is.
Best to ask a gunsmith for help.
is it legal to own a double barrel shotgun in the state of OREGON
If it has a tubular magazine, then the plug is probably inserted in there. When hunting, a shotgun is typically not permitted to be loaded with more than three rounds (including the one in the chamber), and the plug is required to ensure the shotgun is not loaded with more than that. It comes out easily enough.
Remove the cap from the end of the tube and be careful of the spring which is under pressure. The plug will come out with the cap.
Law and Order - 1990 Shotgun 20-10 is rated/received certificates of: USA:TV-14 (V)
You should be able to unscrew the cap on the front of the magazine tube.