I don't know which country you're from, but the normal thing in the UK (where I'm from) is no gun at all, and when I've traveled abroad previously the most was a pistol.
There have been occasional exceptions in the UK, as noted in other comments, but in the case of what I saw in Belgium it was the combination of armed police being a rare experience for me plus the gun in question looking exceptionally large.
I've seen officers with SMGs (or something like that; I'm no expert) in the UK more than once. E.g. in an airport.
Better than that, though, I remember going to a cash machine in India and there was a security guard sat next to it with the longest, biggest shotgun I've ever seen. He gave me an incredibly relaxed nod and smile, and I got my money.
There have been occasional exceptions in the UK, as noted in other comments, but in the case of what I saw in Belgium it was the combination of armed police being a rare experience for me plus the gun in question looking exceptionally large.