Some 90% of 75,000 airguns targeted by tough new laws remain unaccounted for, a shooting sports group has claimed.
The Brocock ME38 airgun can be converted for live ammunition
From Friday owners of airguns which can be converted to fire live bullets need to have surrendered the weapons to police or acquired firearms licences.
The British Association for Shooting and Conservation says owners may dump unlicensed guns to avoid prosecution and wants the deadline extended.
But the Home Office said the law change had been widely publicised.
A spokesman said it was too early to judge how many guns had been accounted for.
"As the deadline has not yet passed, we would not yet have collated statistics from all the relevant police forces on certificates granted or weapons handed in."
He said posters and leaflets had been sent to all police forces in England, Scotland and Wales for distribution, as well as trade organisations.
The Home Office also produced reminder stickers for tins of air gun pellets.
The spokesman added: "We have been continuing to work with the police and any other interested organisations with a view to securing further local and national publicity as the deadline approaches."
Existing owners have until close of business to submit their applications or face a five-to-10-year jail term.
The measures are part of a government crackdown on this type of airgun, which in latest figures, was used in 80 serious crimes between 2001 and 2002.
But the BASC's senior firearms officer, Mike Eveleigh, said the Home Office had introduced "poorly considered knee-jerk legislation, with severe penalties attached, and failed to make people aware of the change".
Mr Eveleigh said thousands of people would now face prosecution and dumped guns could end up in the hands of criminals or children.
"BASC is calling for the deadline to be extended as a matter of urgency, and a temporary exemption from prosecution to be introduced", he said.
Since January, it has been an offence to manufacture, sell, purchase, transfer or acquire any air weapon that uses a self-contained gas cartridge system.
The guns, often known as Brococks after the leading manufacturer, have been reclassified as prohibited weapons as many of them can be easily converted to fire live ammunition.
The ban does not apply to airguns which use a CO2 bulb system.
"The stark reality is that if people ignore the changes in the law they could
ultimately end up in prison," said Chief Supt Paul Robinson, of the Metropolitan Police.
"We know there will be owners of these type of airguns who
want to keep them for legitimate sport but are unaware of the need to license
"We would urge them to apply for a certificate. Anyone else who has a weapon
but does not want to license it should hand it in to a police station straight
The measures have been introduced by the government under the Anti Social Behaviour Act 2003.
Home Office Minister Caroline Flint said the move was part of a range of actions to reverse an "unacceptable" rise in gun crime in recent years.