A method and apparatus are disclosed for substituting a different stock, or a different receiver assembly, in a long gun (e.g., a shotgun) without the use of tools by a user in the field. Applicants' preferred apparatus comprises: a long gun (e.g., an illustrated shotgun) having a stock and a receiver assembly; and an adaptor assembly which removably interconnects the stock to the receiver assembly, wherein the adaptor assembly includes: an adaptor base affixed to the receiver; an end of the stock is mounted onto the adaptor base; and a rotatable locking pin, continuously attached to the base, which can be lifted up and rotated or turned on its axis by 90.degree. to unlock the adaptor assembly. The stock can then be pulled off the adaptor base. Upon a new stock being mounted on the adaptor base, the locking pin can be rotated back to its original orientation and pushed down to lock the adaptor assembly, whereupon the adaptor assembly interconnects the substitute stock with the receiver assembly. While Applicants' locking pin can rotate and move up-and-down, it is always maintained within the adaptor assembly, even throughout the entire process of swapping a different stock.