GT, I think threading is mandated by the toggle mechanism. The Colt/Browning drop link uses a different method of locking the breech bolt to the barrel,the reciever is locked to the breech bolt and the barrel moves. The luger locks via the toggle and reciever is screwed to the barrel.
It could have been engineed differently but then it would have been a 29 Swiss or a Laiti (both still with threaded breeches) :-)
heinz
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