A customer shops for a Christmas present at the National Armory gun store on Dec. 23 in Pompano Beach, Fla. (Joe Raedle/Getty Images)

Three years after 20 children and six adults were massacred at an elementary school in Newtown, Conn., President Obama announced Tuesday a set of modest changes to existing rules on firearms that fall far short of the strict controls he has advocated in the past and that experts say will have a limited effect on gun violence.

"It's a very, very, very small step," said Jay Wachtel, a former agent in the Bureau of Alcohol, Tobacco and Firearms who supervised gun-trafficking investigations in Los Angeles. "This is all an exercise in an illusion of gun control, and to me, it's frustrating."

The most controversial aspect of Obama's executive actions is likely to be new guidance on who must conduct background checks before transferring guns to another person, although administration officials described it as more of a summary of existing law and judicial precedent, rather than a change in policy.

Commercial dealers have to be federally licensed and carry out the checks. Owners selling guns privately -- such as collectors and hobbyists -- usually do not. The administration says the rules will clarify that, in its view, the requirement to obtain a federal license can also apply to small-time dealers, who sometimes claim to be collectors or hobbyists and sell firearms online and at gun shows.

In a conference call with reporters Monday evening, senior White House adviser Valerie Jarrett said that sales by unlicensed dealers claiming to be collectors are a "very, very common way that guns are sold." The administration, though, was not able to say how many dealers are in this category, or how many guns they sell.

Wachtel noted that the bureau typically focuses on licensed dealers who falsify records to divert guns into criminal markets, since those sellers deal in thousands of firearms a year. Prosecuting individual, unlicensed sellers who move only a few guns at a time -- even if they are breaking the law -- would be a waste of the bureau's scant resources for trafficking investigations.

According to the agency, its staff examined the records of just 7 percent of licensed dealers in 2014. Obama will request additional funding for 200 more agents next year, a request that Republicans in Congress will likely reject.

One way that dealers divert weapons is by claiming that a shipment from a distributor never arrived but was stolen in transit. That way, law enforcement has to prove that the guns were in fact shipped and that the dealer sold them illegally.

As part of the executive actions announced Tuesday, the Obama administration will issue a rule requiring dealers to report any guns stolen in transit. This rule, Wachtel said, might make it more difficult for corrupt dealers to evade the authorities -- but then again, they might simply ignore the new requirement and hope that federal agents just never look at their books.

The administration is also hiring more examiners to carry out background checks, and making technological improvements to the system with the goal of handling requests 24 hours a day, seven days a week and processing any request within the statutory limit of three days -- after which a dealer can sell a customer a gun without a check if he has not received a response from the FBI.

[Read more: What liberals don't want to admit about gun control]

Many criminals who use guns to commit homicides don't get them from federally licensed dealers. Instead, a girlfriend or a sibling with a clean record might purchase a gun on behalf of a criminal. Criminals might also buy weapons from friends or contacts on the street, said Harold Pollack, an expert on public health and gun violence at the University of Chicago.

The average gun recovered in a criminal investigation by law enforcement in the United States in 2013 had been purchased at retail more than 11 years earlier, according to federal data. After being legally purchased by law-abiding citizens, those guns might have changed hands many times before eventually entering the gray market.

Pollack was optimistic that by making it somewhat more difficult for owners to transfer their guns without conducting any kind of check on the recipient, Obama's executive actions would reduce the number of guns available on the gray market in the long term.

"I don't think that this is transformative, but I think it’s helpful," Pollack said. "It's making it harder for guns to enter that ecosystem, and will be over time be helpful."

The president also directed federal agencies to sponsor research into technologies to make guns safer, another effort that might improve public safety at some point in the future.