Criminals are using the internet and postal system to get guns into the UK, senior police officers have warned.
MPs will release their report on gun crime on Wednesday
They also told an all-party parliamentary group of MPs there should be more armed officers on the streets as the number of armed criminals grows.
The group is due to publish a report on gun crime on Wednesday.
In their submission to MPs, senior officers said they had seized guns which had been ordered from abroad on internet shopping sites and then simply sent through the post to the buyers.
Their warning follows figures released by the Home Office in October which showed the number of crimes involving firearms had increased by just under 3% in the 12 months to March 2003, to 10,250.
That was well below the 35% rise in the previous year, when gun crime leapt from 7,362 firearms offences to 9,974.
The police officers told the all-party group the system of acquiring guns abroad for use in the UK was largely unpoliced.
A chemical spray gun from Bulgaria converted to fire bullets had been recovered from criminals in Britain, they said.
The group held three evidence sessions this year, gathering evidence from law enforcement officials including those working with Operation Ventara in the West Midlands, Operation Trident in London and the Manchester Gang Strategy Unit.
Chairman and Labour MP Diane Abbott said more people had to be put behind bars if gun crime was to be brought under control.
"We have to look at issues about detecting and conviction much more seriously," she told BBC Radio 4's Today programme.
Ms Abbott also suggested that improved witness protection would go some way towards encouraging people to come forward to testify against criminals.
Conservative home affairs spokesman Oliver Letwin attacked the government's record on gun crime.
Marian Bates was shot in her shop
"Labour promised to be tough on
crime and tough on the causes of crime, but gun crime has doubled under Labour," he said.
"When Labour came to power, they promised to take guns off the streets - they have manifestly failed to do this."
He added that calls by senior officers for more police to be armed should be taken "very seriously".
The MPs' committee also heard evidence from the Police Federation of England and Wales and the Association of Chief Police Officers.
Police say that to help them investigate shooting incidents, hospitals should be obliged to report gunshot wounds.
Four out of 10 police officers feel they lack sufficient armed back-up to deal with the increasing number of crimes involving guns, and the Police Federation wants more firearms specialists to be trained.
A gun amnesty in April was declared a success after 43,908 guns and 1,039,358 rounds of ammunition were handed in.
But Home Secretary David Blunkett faced pressure from the public and opposition parties to tackle gun crime after three shootings a month ago left two people dead and several injured.
Mr Blunkett pledged to do so at Labour's annual conference, after Nottinghamshire jeweller Marian Bates was shot dead by armed robbers as she shielded her daughter at her shop.