I guess Leo knew the same way I know the UK police use Heckler and Koch MP5 sub machine guns, it's just common knowledge.
Isabelle/Theresa wanted rid of nutbar brother and knew he was a nutbar.
Leo thought Pontus was Isabelle's boyfriend and knew he was a copper, maybe he thought she'd stolen his gun and shot Patrik.
Wallander did say something about 'you can't trust your own' IIRC assuming it was a copper who'd shot Patrik.
I'm guessing Sig Sauer's aren't popular among the criminal community.
Kurt didn't know there was any connection between Isabelle and Patrik until after he'd spoken to Leo.
I think that's it anyway, not sure why so many are raving about Wallander on here, it was all far too neat and inter-connected last night for my liking.
I've only watched 3 so I'll continue watching see if I 'get it'.
Oh any comments on the Doc involving the author, not that I was really paying attention.