If this rifle works anything at all like the original prototype, or the later versions of the Johnson, it should be a recoil-operated design, meaning that the barrel has to be able to slide back into the receiver under the action of recoil; you can check this by placing the butt on the ground with the barrel straight up, and pressing down on the barrel. If the barrel slides into the receiver at all, the bolt should begin to move backwards and rotate, so it can unlock from the locking recesses in the barrel. If this is the case, it certainly would be an early prototype, and the barrel should have a longer bearing surface riding inside the receiver tube, instead of the extended receiver of the later models.
PS. MAKE SURE the rifle is empty before you do anything with it.
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