Nikko (ohlin Japan) made guns under the Winchester brand, the Nikko is more than likely a Winchester 101 action, there are some real beauty Nikko's out there as it was suggested they kept the nice wood for their own guns taking into account they were a mid - budget range, the Nikko Shadow Indy's being the higher end, i have a shadow field gun 10mm rib 1/4 and 1/2 and i regularly out shoot expensive guns at clay , pigeon or game, if it suits dont change it, get it serviced , bore it 1/4 and 1/2 and be happy, cartridges now pattern a lot better than the 30 or so years ago when your gun was made and it is not a requirement to hive such a tight choke,