Boko Haram Islamists, whose insurgency has killed at least 20,000 in the northeast since 2009, have long been reputed to have received weapons smuggled from North Africa.
But a recent influx of small arms is also fuelling violence in other arenas, including clashes between cattle herders and farmers in central states.
It is also adding to the plague of kidnapping and cattle-rustling by gangs in the north of the country, and of armed robbery and attacks on oil installations in the south.
"Without the firearms... we would not have the kind of violence of the magnitude we have today," Interior Minister Abdurrahman Dambazau said in January.
In April, Kano state police commissioner Rabiu Yusuf said the solution to illegal weapons was clear: "We need to put them out of circulation."
Doing that is a tall order, however.
The United Nations Regional Centre for Peace and Disarmament (UNREC) has said more than 350 million small arms and light weapons are circulating in Nigeria -- nearly two for every member of its population.
Nigeria accounts for some 70 percent of the 500 million or so weapons thought to be in West Africa, UNREC director Anselme Yabouri told a conference in Abuja in December last year.
Periodic seizures are made: on May 31, soldiers on patrol in the southwest intercepted three trucks carrying boxes of ammunition from Benin.
But with some 4,000 kilometres (2,500 miles) of borders -- most of them porous -- Nigeria has struggled to stem the tide.
Chad, Libya
In Nigeria, there is no legal right to gun ownership and regulation is by the president. Civilians are banned from owning handguns, military rifles or machine guns.
Licences can be obtained from the police for double-barrel shotguns and hunting guns, but permission for this has been suspended because of the security situation.
President Muhammadu Buhari has blamed the 2011 fall of Libyan dictator Moamer Kadhafi and its chaotic aftermath for the rise in the smuggling of prohibited arms.
"Sophisticated weapons" have found their way into the country, ending up in the hands of nomadic herders who previously carried only sticks and machetes, he said in April.
But security analyst Babaji Katagum said blaming the influx on lawlessness in Libya was a "restrictive explanation".
"Chad has for long been an important source of illegal weapons to Nigerian criminals, including Boko Haram," the former Nigerian Army captain told AFP.
Civil conflict has hit Nigeria's military partner in the regional fight-back against Boko Haram several times in the last 30 years.
Weapons from rebel groups have then ended up in Nigeria, said Katagum.
In 1998, troops from Nigeria, Niger and Chad operated as a joint force against arms and drug smuggling in the Lake Chad area where their borders meet.
But it became clear that it was too small to police the vast northern frontier effectively, particularly after its mandate was later expanded to tackle Boko Haram.
An aerial surveillance unit -- the Air Border Guards -- was set up in 2007 comprising Nigerian immigration, customs and air force personnel.
But it was grounded just five years later by mismanagement and internal rivalry.
Repeated seizures
As well as overland routes, Nigeria's sea ports have been used by cartels to import weapons, by declaring containers as household goods or construction material to evade detection.
In September 2017, customs officers in the commercial capital, Lagos, seized 470 pump-action shotguns labelled as plumbing supplies in a container from Turkey.
That brought the number of weapons seized in the first nine months of last year to nearly 2,700, said the comptroller-general of customs, Colonel Hameed Ibrahim Ali.
Last month, the head of Nigeria's federal police, Ibrahim Idris, said some 4,000 illegal firearms were recovered across the country in just three months.
Officials also publicly destroyed nearly 6,000 guns surrendered by about 3,000 kidnappers and cattle thieves as part of an amnesty in northern Zamfara state.
But Katagum said such discoveries were an "insignificant number" compared to the huge numbers of weapons in circulation.
"All these seizures are only an insignificant fraction of the huge flow of illegal weapons that pass through our ports into the country," a customs officer said, speaking on condition of anonymity.
"We don't have the capacity to check the contents of all cargo container imports -- we rely on intelligence and our instincts."
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