That's a common target for net payable to the shareholders, at least it was when I was in the business. I think most gun stores, even those in large efficient "buying groups" operate on less gross margin than most retailers. Based on wholesales prices I've seen and the retail mark-up I'd guess about 20% average depending on the product manufacturer and type of product. Some go as high as 50% GPM, but that's rare.
True, their Gross Margin (Publix at least) runs about 28% and their net margin is 2.5%. They do have much a higher cost structure than a gun store though, and a gun store doesn't have to worry about a Glock spoiling after the expiration date! I wouldn't group Grocery stores in with retail though when talking about margin.