Not at Costo here in the home of Costco, Walmart, but ammo is iffy, and we even have a local Kroger store (fred Meyer, here in the PNW) with both guns and ammo.
Costco in the Phoenix area is posted no weapons....was in the Sacramento area in the recent past, went to Costco with the FIL.....paid special attention to the entry...Costco was not posted as a gun free zone......what gives with that??