Both Arms Peddler and Jormungand (as you can probably guess) are about weapon merchants. Both of those manga also feature a young boy who joins an older female weapon merchant and thats all you need to know from this recommendation.
Both Kiba no Tabishounin and Jormungand have a young kid who joins with an older woman and travel the world (in high action ways). In both, the older woman just so happens to be an arms/weapons dealer who give musings upon their work.
Rokuro Okajima is a small-time salaryman who is carrying documents for his company, when the ship he's traveling on is attacked by pirates. Kidnapped, he discovers to his dismay that his employers' main concern is to ensure the documents don't get into the wrong hands, even if it means sending the carrier to the bottom of the sea. Now, with his former life ruined and his kidnappers seeming comparatively friendly, "Rock" decides to join their merry band of mercenaries, and sets out with a new career to the shadier corners of the South China Sea.
Both Black Lagoon and Jormungand are manga about a group of mercenaries of sorts. Lagoon Company in Black Lagoon does jobs ranging from smuggling, pirating to escorting and all of those contain a fair amount of gunfights. The group from Jormungand is in arms trading business, so what they mainly do is smuggling and theft (not so far from pirating). I don't think I need to mention that those also involve lots of gunfights. If you think that such groups can only consist of people who can't really be called normal, than your right on your money. Anyway, as you can see those series are similar, so you really should go read one if you liked the other.
They both involve a young male character, who grows up in a corrupt occupation. One being drug dealing and the other being arm dealers. Both characters gain skills in fighting and killing in order to live and surrounded by more skilled killers.