I have dealt with FOC a couple times, no issues at all. As a modern retailer, I would be very surprised if they didn't take the rifle back without hassle.
Retailers get to do their own thing though and they deal with the consequences too.
IMO, it's not a SALE unless the goods being exchanged are the agreed upon items. So if the case described is accurate, The SALE isn't final because it hasn't been completed yet.
Since the buyer has yet to receive the item they purchased, no SALE has actually been completed. The incorrect item is not even relevant to that equation.
In other words, the buyer is within their rights to demand the correct product, even if they don't return the incorrect one.
Under BC's Sale of Good's Act: (Not sure how it works elsewhere in Canada but likely similar (basically word for word in Ontario) and in the case of interprovincial sale, both would likely apply anyway)
Buyer's right of examining goods
38 (1)If goods are delivered to the buyer that the buyer has not previously examined, the buyer is not deemed to have accepted them unless and until the buyer has had a reasonable opportunity of examining them for the purpose of ascertaining whether they are in conformity with the contract.
(2)Unless otherwise agreed, when the seller tenders delivery of goods to the buyer, the seller is bound, on request, to afford the buyer a reasonable opportunity of examining the goods for the purpose of ascertaining whether they are in conformity with the contract.
Buyer not bound to return rejected goods
40 Unless otherwise agreed, if goods are delivered to the buyer and the buyer refuses to accept them, having the right so to do, the buyer is not bound to return them to the seller, but it is sufficient if the buyer intimates to the seller that the buyer refuses to accept them.