There are two pinned versions, and one threaded. The threaded barrel will have a larger shoulder and wrench flats. The standard pinned barrel will not have the same shape as the threaded one in front of the receiver, and there will be the pin. This pinned barrel is not a problem. Then there is the problem pinned version - there is a threaded shank set into the receiver, and the barrel is pinned to this sleeve. This is the version which has given problems. Not very many of these around, perhaps only one batch imported. The first two versions are fine.