Many switch barrel rifles use a screw to keep the barrel from turning out of the action but these barrels also have a shoulder to put the threads in at least some tension. Most of these type setscrews are threaded tangentially to the receiver ring in such a way that they bear on a milled portion of the barrel tenon pushing the barrel in the "tighten" direction.
You could lock a nutless and non-shouldered barrel to a receiver by milling a slot through the receiver ring and then welding two opposing lugs spanning this slot through which one or two socket head capscrews would serve to compress the slot and lock the barrel in with the threads in interference.
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