The number illegally owned is literally unknown - although it is known the numbers are rising with senior police officers such as Sir Paul Condon, the Metropolitan Police Commissioner, expressing fears of an emerging "gun culture" on the streets of Britain's inner cities. Estimates of illegal weapons range from 500,000 to a million or more, perhaps many more.
The Dunblane tragedy is bound to lead to calls for tighter gun control. But what more could be done - short of banning all gun ownership outside the hands of the armed forces and police - to tighten up what its advocates argue is already one of the world's tighter gun regimes?
The Hungerford massacre in 1987 led to the last significant tightening of British gun law. Then Michael Ryan, 27, killed 16 and wounded 13 before killing himself after going on the rampage with weapons which included a semi-automatic Kalashnikov AK47, one of three handguns and two rifles, the other a .30-calibre MI semi-automatic carbine which he legally owned.
The result was amendments to the 1968 Firearms Act. Ownership of high- powered self-loading rifles and burst firing weapons was made illegal, while pump-action shotguns with a magazine of more than two bullets were classified as firearms, not shotguns, requiring a tougher certificate.
It didn't stop the killings. In 1989 Robert Sartin, a 23-year-old civil servant suffering from schizophrenia, stalked the streets of Monkseaton in Whitley Bay, Tyneside, with a shotgun - killing one and wounding 16.
Since then, there have been smaller, less dramatic incidents, contributing to a total of between 50 and 71 people who have died each year in shootings of one kind or another.
Hungerford, however, brought a changed attitude to guns by the police, according to the British Association for Shooting and Conservation, a key part of the gun lobby. Checks are tighter. Applicants have to show good reason for wanting a gun - usually target shooting or other membership of a gun club, or a farmer's requirement to deal with vermin. Any past history of mental illness has to be declared and a counter-signature is required from a person of standing declaring the applicant to be of good character.
Significant numbers among the one million or so people who shoot - anything from air pistols, to Olympic gunmen to wildfowlers and deer hunters - have been driven from the sport, according to Robin Peal, the BASC's head of public affairs - "people who couldn't be bothered with the hassle".
The result has been a fall in the number of gun certificates on issue - down to 670,000 at the end of 1994, almost a quarter fewer than in 1988 and the lowest number since 1971. Firearm certificates totalled 140,200 in 1994 - marginally more than in 1992 but 15,000 down on the 1988 total. Certificates can cover more than one weapon.
It is not, the gun lobby maintains, the legal guns that are the problem. "The numbers are tiny compared to uncertificated weapons, where the estimate is that there are between two million and four million guns out there," Mr Peal says. "It is the ones the police don't know about that are the problem."
And their numbers have been growing. They range from ancient muskets to Second World War revolvers, to a recent flood of weapons from Eastern Europe following the break-up of the Soviet Union that has led police to discover sub-machine guns and other automatics that could not be held legally on a firearms certificate.
Illegal "armourers" will even hire out guns for crime - from around pounds 300 for an ageing revolver to pounds 700 for a modern semi-automatic handgun. The exact price, Mr Peal says, "will depend on whether it is returned dirty or clean" - fired or unfired - the higher price reflecting the fact that fired ammunition can be traced to the gun, making it "hot", and in need of disposal.
So what more can be done? Bill Tupman, until recently director of the Centre for Police Studies at Exeter University, said yesterday that any decision about gun control "involves balancing risk against cost. When 13 children are killed, what is the cost of those lives against the risk of it happening?"
After Hungerford, Mr Tupman undertook a study of gun control for the BASC, when it seemed that Hungerford "was simply a one-off". But after the Whitley Bay shootings in 1989 there were smaller incidents, and now Dunblane. He said: "It seems someone is going berserk with a gun around once a year now.
"What is being asked for is a system which prevents someone going off their head and misusing guns. There are only two ways to do that. One is to require people who hold guns to go to the doctor for an annual certificate of fitness to hold a firearm. The other is for gun clubs to inform the police whenever someone they know who is in possession of firearms is unstable."
The latter they should do anyway, he argued, and the time might be coming for the former. "Police officers who carry guns have to be checked twice a year, facing psychological testing. If the police have to do that, at what point do we start to demand the same of anyone who carries weapons which are capable of lethal force?"
The gun lobby's objection, he said, would be the cost - "and it would be incredibly costly. But the cost has been incredible for the parents of those 13 children. The argument for such a move is strengthened by the growing number of deaths."
Similar ideas surfaced after Hungerford - as the picture emerged of Michael Ryan as an odd-ball loner, in his very different way as much a misfit as Thomas Hamilton, the author of yesterday's massacre, appears to have been. A small man, with a deep grudge and an obsession with firearms.
But the most powerful opposition came from doctors - in the shape of the British Medical Association - which resisted the idea. A spokesman for the BMA said yesterday: "It was put to us that doctors should provide some sort of 'sanity certificate' for someone seeking a shotgun or firearms certificate, but our view is that it is frankly impossible for a doctor, particularly a GP, to do that."
Past mental illness is taken into account in issuing certificates, but beyond that "it is virtually impossible for a doctor to make a judgement about someone's fitness to hold a gun," according to the BMA. Doctors were also worried about what would happen "if they provided a certificate and the individual then went out and shot a lot of people. Would the doctor, somehow, be held responsible for a judgement he could not really make?"
And given the number of certificates issued annually - each is renewable every three years - "it would be frankly impossible for psychiatrists to provide a full psychiatric examination of everyone holding a certificate, and there would be no guarantee even then that you would spot the people at risk".
The practicality of psychological - as opposed to psychiatric - examinations to identify possible mental illness - was also doubted by Gerard Bailes, a forensic psychologist specialising in firearms who works with the Norfolk constabulary's armed officers.
Psychological testing is used in training and re-training, he said - but to help identify officers who will react well under the specific stress of using firearms during police work. "I don't think a psychological test exists that would pick up this sort of risk," he said of Dunblane.
By coincidence, the Firearms Consultative Committee - a Home Office sponsored body which includes the police and shooting interests, is meeting today and such issues are bound to come again on to its agenda in the wake of the killings.
The gun lobby, however, will resist. Ian McConchie, general secretary of the National Pistol Association, said he shared the horror and shock at Dunblane but would oppose "knee-jerk" calls to further tighten gun controls. "However tight you draw the law, it will never protect against someone just going over the top and losing their marbles." Perhaps, but that will not console nor persuade the parents of Dunblane.
The right to keep and bear arms has rarely been out of the news recently. In 1994, the House of Representatives voted to outlaw the ownership of 19 types of assault weapon previously available. However, the US still has notoriously high homicide rates, and gun laws range state-by-state from lax to almost non-existent.
The "gun lobby" is extremely strong in the US, and a telling debate has been going on in this sector of society recently as to whether "the right to bear arms" should be taken in a strictly literal sense. One side argues that if a weapon is not too heavy to be borne (that is, lifted), it should be freely available to the population. In 1994, there were 600,000 incidents in which guns were used "defensively", and firearms deaths in the US average 40,000 per year.
Almost every able-bodied man up to the age of 32 is a member of the Swiss army reserves, and maintains a gun of some sort. Many men choose to buy their gun when they leave the reserves.
Moreover, fears were recently expressed that private gun laws were so lax that Switzerland would become a major supplier of arms to the former Yugoslavia. These laws are now under review.
The homicide rate is roughly a quarter of that in the UK. Application can be made for permission to own a gun but in nearly all instances will be refused. For this reason, gun ownership statistics are not available. In 1993, 1,672 illegally held guns were seized by Japanese police.
Recent massacres by deranged gunmen in France, along with European legislation, have led to a tightening of gun law in France. Private ownership of handguns is now forbidden, but hunting rifles may still be bought by those who hold a hunting licence. Owners must also register their ownership of a hunting rifle with the police.
Since 1987, when the Hungerford killings took place, border checks throughout Europe have eased considerably and borders to the former Communist bloc have become less restricted. Consequently, the whole of Europe has seen a booming illegal market in guns of all sorts, irrespective of new national or European gun control laws.