In late March, the CBSA found the gun in Canada Post mail destined for Dawson Creek, which is 260 kilometres northeast of Prince George, near the Alberta border.
Working with Canada Post, police tracked the alleged recipients of the package and arrested two Dawson Creek men, aged 31 and 40, who are both well-known to police.
Both men are facing charges of importation of a prohibited firearm.
Const. Lesley Smith says police are finding more illegal goods are being sent by mail.
"We are finding more often now that suspects are using the Canada Post mail services to either transport firearms, [or] other illegal activities, as well as drugs, back and forth to each other. So we are working with Canada Post in relation to these other cases," she said.
Dawson Creek RCMP said through a search warrant connected to the gun in the mail, police also found and seized a homemade silencer, additional firearms and ammunition.
Both of the men have been released on bail, police said.