Everyone knew there would be some gun owners flouting the law that legislators hurriedly passed last April, requiring residents to register all military-style rifles with state police by Dec. 31.

But few thought the figures would be this bad.

By the end of 2013, state police had received 47,916 applications for assault weapons certificates, Lt. Paul Vance said. An additional 2,100 that were incomplete could still come in.

That 50,000 figure could be as little as 15 percent of the rifles classified as assault weapons owned by Connecticut residents, according to estimates by people in the industry, including the Newtown-based National Shooting Sports Foundation. No one has anything close to definitive figures, but the most conservative estimates place the number of unregistered assault weapons well above 50,000, and perhaps as high as 350,000.

And that means as of Jan. 1, Connecticut has very likely created tens of thousands of newly minted criminals — perhaps 100,000 people, almost certainly at least 20,000 — who have broken no other laws. By owning unregistered guns defined as assault weapons, all of them are committing Class D felonies.

"I honestly thought from my own standpoint that the vast majority would register," said Sen. Tony Guglielmo, R-Stafford, the ranking GOP senator on the legislature's public safety committee. "If you pass laws that people have no respect for and they don't follow them, then you have a real problem."

The problem could explode if Connecticut officials decide to compare the list of people who underwent background checks to buy military-style rifles in the past, to the list of those who registered in 2013. Do they still own those guns? The state might want to know.

"A lot of it is just a question to ask, and I think the firearms unit would be looking at it," said Mike Lawlor, the state's top official in criminal justice. "They could send them a letter."

An aggressive hunt isn't going to happen, Lawlor said, but even the idea of letters is a scary thought considering thousands of people are now in an uncomfortable position. At the least, the legislature should reopen the registration period this year with an outreach campaign designed to boost the numbers.

It could be a tough sell. On Thursday night, Guglielmo heard from a constituent at a meeting in Ashford, who said most of his friends with military-style rifles such as AR-15s had not come forward.

"He made the analogy to prohibition," Guglielmo said. "I said, 'You're talking about civil disobedience, and he said 'Yes.' "

But it's not just refusers. A reopened registration would help many who failed to come forward out of ignorance.

"There are a lot of people, they just do not know about this law," said Scott Wilson, president of the 12,000-member Connecticut Citizens Defense League, a Second Amendment advocacy group. "There are people finding out now after the fact."

The law was widely covered in the media and Wilson said his group sent information to its members. But gun owners can be an independent bunch.

Guglielmo, who voted against the sweeping gun control bill, said he intends to raise the concern at the next meeting of the public safety committee. Lawlor said Gov. Dannel P. Malloy's administration is willing to talk about solutions.

But Lawlor, an undersecretary in the state Office of Policy and Management, said that even if many thousands of guns remain unregistered and are now illegal, the law is not necessarily failing at its goal.

"Like anything else, people who violate the law face consequences. … that's their decision. The consequences are pretty clear. …There's nothing unique about this," Lawlor said. "The goal is to have fewer of these types of weapons in circulation."

The law was adopted after the December 2012 massacre at Sandy Hook Elementary School. Its main provision was a dramatic expansion of guns classified as assault weapons banned for sale in the state. The ban now includes any semiautomatic firearm — that is, one that reloads a round after each pull of the trigger — if it has even a single military-style characteristic, such as a pistol grip.

Any semiautomatic firearm banned for sale could remain legal if its owner registered it by Dec. 31. Those that were made before the state's first assault rifle law in 1993, and were not deemed to be assault weapons in that law, do not have to be registered.

The AR-15, a type of rifle, not a brand, is among those that must be registered and represents 50 percent to 60 percent of all rifle sales in the United States in recent years, federal figures show.

Sorting out the number of potential new felons is a guessing game. State police have not added up the total number of people who registered the 50,000 firearms, Vance said. So even if we knew the number of illegal guns in the state, we'd have a hard time knowing how many owners they had.

And it's all made even harder by the fact that the April 2013 law also required owners to declare ammunition magazines that hold more than 10 rounds, under penalty of a felony charge. Magazines are impossible to track because they have no serial numbers. The state received 36,932 declarations of these high-capacity magazines by the deadline, Vance said. An additional 1,277 were sent back for more information.

Each of those magazine declarations is a person, not a magazine. The state police firearms unit has not added up the number of magazines, Vance said.

As for the numbers out in circulation, in 2011 the nonpartisan Office of Legislative Research said there were more than 2 million high-capacity magazines in Connecticut, based in industry information from the National Shooting Sports Foundation.

That same legislative report cited a foundation estimate that there were, at the time, 1.2 million rifles in Connecticut, based on national sales and manufacturing figures. Of those, 372,000 used high-capacity magazines, making it very likely that they were in the class of weapons affected by the 2013 law. There are some rifles using high-capacity magazines that are not classified as assault weapons in Connecticut, but not many, said Jake McGuigan, director of government relations at NSSF.