I just want to point out that at least here in the south of Sweden, no one under the age of seventy says 'smörgås'. We say 'macka'.
The only exceptions are in words like smörgåsbord (buffet) and smörgåstårta (tårta means cake; it's a common thing to eat at for instance christening parties (it's easy to make lots of); it's basically white bread in layers with fillings (hence 'tårta/cake') that can vary a lot but usually has smoked salmon and shrimp, or ham and roast beef, with lots of vegetables and often mayonnaise to keep it from getting dry) In those cases you can NOT say macka. Macka is just ordinary sandwich.
We also use more specific words (which are borrowed), like baguette and toast. Bun is called bulle or sometimes fralla.
It's correct with "en smörgås" Unfortunately, Swedish has very few rules for en/ett sometimes it does not make much sense (animals are supposed to be "en" but you say "ett lejon"). Also some words can be both. Like most people would say "en apelsin " but in the far south, you can hear "ett apelsin" because apelsin is an old shorthand for "äpple från kina".