I'd say it depends on just what the markup on the gun IS, considering most shops don't have much of a markup, that would be a bit more of a dip into profits, would it be a nice thing to do though? Yes.
IF the store can afford to give those discounts, then yea, they should go for it, if that's going to drastically harm their bottom line though, well they have to pay their workers and eat themselves, so in that case, I'd say no.