Four more people have been arrested in raids targeting the supply of firearms on Merseyside.
Five properties were searched in Wirral and a man and a woman - both in their 20s - were arrested on suspicion of conspiring to import firearms.
More than 50 firearms and 80kg of heroin were seized in earlier raids.
A woman in her 20s was held on suspicion of money laundering and a man in his 50s was arrested on suspicion of possessing Class A drugs.
Last month nine suspects were arrested in Wirral and four more were detained in Bulgaria.
Two packages destined for Merseyside were intercepted. They contained 14 firearms, eight silencers and 691 rounds of ammunition.
In co-ordinated raids, Bulgarian authorities seized 39 handguns, 21 silencers, 1,253 rounds of ammunition and 80kg of heroin all destined for the UK.
The operation, led by the National Crime Agency (NCA), was in partnership with Merseyside Police and Bulgarian law enforcement.
NCA Branch Commander Paul Risby said: "The recovery of the 56 firearms was a significant triumph in our fight against gun trafficking into the UK.
"And though we have hugely disrupted the organised crime group there is still work to do."