The intense battle over universal – or not-so-universal – background checks for firearms purchases further illustrates the deep cultural divide over guns, pitting those who see the checks as a profoundly obvious common-sense step to stem the supply of deadly weapons to criminals and the violence-prone against those who see it as an expensive hassle that will inconvenience legitimate gun owners while doing little to keep guns out of the wrong hands.

But what’s involved in a background check anyway?

The process – currently conducted whenever a federally licensed gun dealer sells a gun – begins with Form 4473, a Bureau of Alcohol, Tobacco, Firearms and Explosives record that gathers demographic information and asks a series of questions to determine if a would-be buyer is barred from possessing a gun.

Perhaps the most significant  is the first, asking: “Are you the actual transferee/buyer of the firearm(s) listed on this form?” That question is designed to ferret out “straw buyers”  – individuals with clean records who are buying a gun on behalf of another person who cannot legally acquire weapons. Gun store owners have told tales of young women coming in with detailed sketches of the store’s display cases and then pointing to a weapon matching a spot marked on the sketch – a scheme, the owners presume, to obtain a weapon for a boyfriend with a criminal record.

It is illegal to falsely answer the straw-buyer question, but it is still self-reported and the intuition of the store owner is the only guard against a lie.

The form then asks a series of questions to determine if the buyer is prohibited from owning a gun. “Have you ever renounced your United States citizenship?” “Are you an alien illegally in the United States?” “Are you a fugitive from justice?”

Buyers are also asked if they have ever been convicted of a domestic violence crime or a felony or any criminal charge that carried a possible sentence of more than one year in prison. Other questions ask about restraining orders, drug use and commitments to mental hospitals.

Those answers, too, are self-reported, but licensed gun dealers don’t take a buyer’s word for it. Instead, they are required to submit the buyer’s name to the National Instant Criminal Background Check System, known as NICS. The system, managed by the FBI, searches criminal records and other sources to determine if the prospective buyer is disqualified from owning a gun, typically for the reasons listed on Form 4473.

The background check is often nearly instantaneous. But if the check turns up a possible match to a prohibited person, the system will instruct the dealer to delay the sale while the information is checked. If a resolution is not reached within three days, the dealer has the option to go ahead with the sale or wait for authorities to complete their investigation.

The NICS check was mandated by the Brady Act in 1993 and went live five years later. In the more than 14 years since, the FBI says 166 million NICS checks have been performed, resulting in more than 1 million federal denials, most often as a result of criminal convictions. In Connecticut last year, 237,496 background checks were performed.

That background check is not required when individuals who are not federally licensed gun dealers sell weapons informally or at gun shows or when weapons are transferred between relatives. This week, two U.S. senators, Republican Pat Toomey of Pennsylvania and Democrat Joe Manchin of West Virginia, released a proposal to extend background checks to all sales at gun shows, over the Internet and between strangers, while continuing to exempt “personal transfers” between relatives and friends.

That proposal brought a swift rebuke from gun-rights advocates, who have generally pushed back against expanding background checks. The Newtown-based National Shooting Sports Foundation, the gun-industry trade group, opposes expanded checks, while simultaneously calling for improvements in the NICS system. Currently, states are inconsistent in the quality of the data they report to the system, and the foundation advocates pressuring states to provide more information.

“Before we talk about requiring background checks on private party transfers, we should  fix NICS,” the group says.

But what’s the harm in running more background checks, even if through an imperfect system? The NSSF says the proposal will burden licensed firearms dealers who will have to run the checks for private sellers, possibly for a mandated fee that doesn’t cover their costs. Dealers also fear that expanded checks will overtax the system and lead to delays in completing their own sales.  They also share a fear among some gun enthusiasts that the government might use the system to compile a registry of gun owners in the country, which some believe could be used by tyrannical leaders to disarm the nation. In a nod to those worries, federal law requires the government to destroy any record of a NICS check by the next business day.

Gun-control advocates say those are small prices to pay to limit the flow of guns to those barred from owning them.

“This bill will not only help keep guns out of the wrong hands, it will help save lives and keep our communities safe,” New York Mayor Michael Bloomberg said in a statement.

Countered the National Rifle Association: “Expanding background checks at gun shows will not prevent the next shooting, will not solve violent crime and will not keep our kids safe in schools,” the group said. “The sad truth is that no background check would have prevented the tragedies in Newtown, Aurora or Tucson.”

That pitched battle now moves to the Senate, where members voted Thursday morning to block a filibuster and move forward with consideration of gun legislation.