Who can use a blunderbuss?

I'm a bit new around here and to Mordheim as well, so forgive me if this is obvious to everyone else, but who can use a blunderbuss?

It looks to me as if only Mercenary marksmen and Pirate gunners are able to be equipped with. And other heroes from any warband can be equipped with them after taking the skill Weapons Expert. Is this correct?

In searching and reading through some other topics about blunderbusses, it seems almost like some people have entire warbands equipped with them, or that their whole campaign has spiraled into a blunderbuss war. Here's a quote: "They play such a big roll in our campaign because people have been buying archers and just giving them blunderbusses. My warband has 4, another player has 4 and a third player has 6." Must it be that they are all playing as Mercenaries/Pirates?

Yep, if you don't have it on your list you need training. I haven't come across many pirate warbands, but mercs are often popular, so, yeah, they were probably all mercs or pirates or they gave 4 heroes Weapons Expert.

Your assumptions about who can use a Blunderbuss are correct. There are fan-made warbands that allow some heroes to be equipped with a blunderbuss without Weapons Expert, but if you limit your warband choices to '2005 Rules Review official' and 'those found in Town Cryer' then its just the warriors you have already identified and heroes with the Shooting skill Weapons Expert.

The quote above comes from this thread--
https://boringmordheimforum.forumieren.com/t6208-the-problem-with-blunderbuss

The OP on that thread mentions that he plays "mercs" as does at least one of his friends. (I got the impression that the whole group did). "Mercs" is a pretty common way of referring to Empire Mercenary warbands from the Mordheim Rulebook.

Either way the posts on that thread should make it clear that the OPs problem was pretty much created by how his group played the game and not all that widespread.