Most of the mainstream chain supermarkets in the US don't sell guns (Kroger, Target, Whole Foods, Costco, Albertsons, Ahold, Publix, Sams, HEB, Meijer, Aldi, et al.). Walmart is the primary exception there.
Overwhelmingly that's now handled by independent gun stores, or a select few sporting good chains (eg Dick's Sporting Goods - the largest sporting goods chain in the US - which sells a restricted set of rifles and a few shotguns; they came under pressure not long ago to get rid their guns and they capitulated and reduced what and how they sell [1]).
Big corporations in the US are drastically more sensitive today to gun issues and the related bad PR that goes with selling guns (even Walmart has rolled back their selling of guns).
[1] https://finance.yahoo.com/news/why-dicks-sporting-goods-deci...