The FamilyMart tamago sando is the convenience-store egg sandwich as one chain's house recipe, and the rivalry is the reason it has a name of its own. The form is the standard Japanese tamago sando, egg salad between two slices of crustless soft white bread, but in Japan the konbini egg sando is a category people argue about, and FamilyMart's is the entry partisans line up against the 7-Eleven and Lawson versions. What defines this one is not the idea, which every chain shares, but the specific FamilyMart recipe: its own mayonnaise balance, its own egg ratio, its own bread, tuned and held constant across thousands of stores so the comparison is even possible. The egg salad and the shokupan need each other the way any egg sando's do; the achievement here is a fixed flavor that lands the same in every prefecture.
The craft, such as it is, lives in the formula and the handling. The filling is chopped egg bound in a tangy, umami-forward Japanese mayonnaise, the proportions set so it is creamy and well seasoned without weeping into the bread, and FamilyMart's mix is pitched to its own house point on the sweet-to-tangy scale that regulars can pick out blind. The shokupan is soft, fine-crumbed, and crustless, cut to a clean rectangle with the salad spread fully to the edges so no corner is dry and the cross section is even. Kept properly chilled, it holds texture for its shelf life. A good one tastes clean and balanced straight from the cold case and matches the customer's memory of the last one. A poor one has sat too long or been stored warm: the bread turns tacky and the filling goes pasty and flat, the exact failure the standardized recipe exists to rule out.
The comparisons are the variations. The 7-Eleven and Lawson tamago sando run the same outline at their own recipe points, and tasting the three side by side is a national pastime. The grilled rolled-omelet tamago sando from specialist shops and the premium omiyage versions in airports are different textures and different ambitions. Each of those deserves its own article rather than being crowded in here.