We can trace guns that have been used in crimes to unscrupulous gun dealers that may be selling to straw

purchasers and dumping them on the streets.

вообще не пойму, как перевести в данном контексте dumping them on the streets. недобросовестные продавцы оружия продают подставным покупателям, и дальше что? 

to dump - стрелять из пушки

Вроде так

A straw purchase occurs when one person buys a gun for someone else who is either prohibited by law from possessing a gun or who does not want his or her name associated with the transaction. A straw purchase is a felony with a punishment of 10 years in jail and a fine of up to $250,000.

по dump есть сомнения

вряд ли продавцы оружия стреляют в своих покупателей

-dumping on the sreets 

-вываливают это оружие на наши спокойные улицы

'Stover was what's known as a "straw purchaser": a woman with a legitimate licence to buy guns — but who actually purchased them for people who can't legally do it themselves.'

То есть перекупщик, который распространяет это оружие в темных кварталах улиц тем, у кого лицензии или разрешения нет.

...в результате чего оно [оружие] отказывается в руках у уличных преступников

Правильно думали.

Никто не знает почему on the streets?

n.

1. a person whose function is only to cover another's activities.

Подставное лицо.

in the streets ... suggests that the walker is not using the pavement/sidewalk intended for pedestrians but walking in the roadway used by the traffic. One frequently comes across  children playing in the streets . the expression  on the streets  (without walk) is common, referring to life/events that take place on the streets. https://ell.stackexchange.com/questions/123022/he-was-walking-through-on-in-the-streets-of-the-city

Note also 'to sell drugs on the streets' = to sell drugs openly and in an uncontrolled way to anyone who'll buy them.