Time will tell, but my sense is that the new MCC code will be more limited in its application. It looks like Sportsmans Warehouse generates less than half its revenue from firearms/ammo sales, so I doubt that it would be forced from the broader sporting goods MCC code to the new firearms-specific one. Traditional gun stores which generate a very high % of revenue from firearms/ammo are where I see this MCC being applied.
There is also a merchant processor convention for providing line-item detail for transactions (level III data) that may get pressed into use for those sorts of retailers, so that a card issuer could discriminate between firearms/ammo purchases and general sporting goods purchases at a retailer that sells both. I suppose this could also be applied by gun stores to differentiate between buying a gun and buying a gun safe, but I doubt mom and pops will go this route.
We will see how this plays out, but there is a lot of work to get this scheme fully implemented. One card issuer accepting the new MCC code doesn't do a thing until the retailers are converted over and reporting it. From what I have read, it is not as easy as just changing an MCC code in a data field to affect the change at the processor level. My guess is that, even if the entire industry moved in that direction in lock step, it would take years to fully implement.