Over three months after a gunman killed 26 at Sandy Hook Elementary School in Newtown, Conn., state legislators announced a bipartisan compromise Monday on new gun restrictions.

The compromise, which State Senate Majority Leader Martin Looney called the most comprehensive in the nation, comes after months of contentious negotiations and is likely to be voted on this week. It includes new bans on the sale or purchase of high capacity magazines and certain semi-automatic assault weapons, a registry for frequent weapons offenders, universal background checks and a requirement of a gun permit to buy ammunition. To the disappointment of gun-control advocates, however, the deal includes a “grandfather” clause, allowing those already in possession of the newly banned assault weapons or magazines capable of holding more than 10 rounds to keep them, provided they submit to a registration process.

“Some of the suggestions are good, and some we were disappointed a little bit — especially on the magazines. We wanted them to be eliminated totally,” Lt. Gov. Nancy Wyman told the News Monday. “But a lot of the bill is good.”

The compromise would expand the state’s existing assault weapons ban from 60 specific weapons to over 160, including the Bushmaster AR-15 used by Adam Lanza in the December shooting, in addition to altering the definition of an assault weapon. Under the new ban, weapons with only one, rather than two, of the military characteristics specified in the law would be illegal to buy or sell.

Should the legislation be signed into law, the registration for high-capacity magazines and assault weapons would be the first of its kind in the country. Owners would be required to register the weapons with the state’s Department of Emergency Services and Public Protection by Jan. 1, 2014. Furthermore, they would be prevented from loading the magazines with more than 10 rounds outside their home or licensed shooting ranges.

Bans on weapons frequently used in mass shootings are only one part of the compromise — the deal would also create a “dangerous weapon offender registry,” which would allow law enforcement to keep track of the whereabouts of individuals convicted of weapons offenses for five years after their release from prison.

A new requirement of a gun permit to buy ammunition and an expansion of background checks to cover all gun purchases are also central to the legislation. Under current law in Connecticut and throughout most of the country, private sales of guns, including sales at gun shows, are unregulated. Closing the “gun show loophole,” as it is frequently called, has long been a goal of gun-control advocates.

In perhaps the most pointed reference to the causal circumstances of the Newtown shooting, the compromise significantly expands the scope of the state’s firearms safe storage law. It would broaden requirements to store a firearm securely to instances in which the owner “knows or should know” that anyone likely to gain access to the firearm is ineligible to own a gun or poses a risk of injury to others or themselves.

Lanza’s mother purchased the guns used in the Newtown shooting legally but did not secure them.

The deal comes after months of wrangling in the legislature, largely focused on the ban on high-capacity magazines and assault weapons, and represents a major shift toward bipartisanship. In February, Democrats and Republicans on the Bipartisan Task Force on Gun Violence Prevention and Children’s Safety, which the legislature established to develop policy recommendations in the wake of Newtown, released separate proposals. The Democrats’ plan included the ban, while the Republicans’ did not.

Even though Republicans played a leading role in the negotiations, hesitation on the bill’s more controversial provisions was common among Republicans Monday.

“The caucus is split on the bill,” said Pat O’Neill, a spokesman for Republicans in the Connecticut House of Representatives. “They have mixed emotions on it.”

O’Neill noted, however, that State House of Representatives Minority Leader Lawrence Cafero, who played a leading role in the negotiations, plans to vote for the bill and is actively encouraging other members of his caucus to do the same.

Early reports of the compromise, however, left some gun-control advocates, including several families of Newtown victims, disappointed. On Monday, relatives of some of the victims gathered to demand an “up or down” vote on high-capacity magazines, which they contend should be banned entirely.

“We learned, the way that no other parents should learn, that the most dangerous, dangerous part of an assault weapon is the magazine,” Nicole Hockley, whose six year-old son, Dylan, was killed in the Newtown shooting, said at the gathering. “The horrible, brutal truth is that 154 bullets were fired in four minutes, killing our children, our daughters, our wives … We have learned that in the time it took him to reload in one of the classrooms, 11 children were able to escape. We ask ourselves every day — every minute — if those magazines had held 10 rounds, forcing the shooter to reload at least six more times, would our children be alive today?”

Neil Heslin, whose son Jesse was also killed in the shooting, said he thinks that a registration process would be impossible to enforce, as the magazines do not have serial numbers.

But Ron Pinciaro, the director of Connecticut Against Gun Violence, expressed support for the bill, citing the nature of compromise and political realities. He told the Hartford Courant Monday that his organization, which has frequently been at the fore of calls for new legislation, would not push the up or down vote requested by the Newtown parents.

Gov. Dannel Malloy expressed similar dissatisfaction with the bill, although his communications director, Andrew Doba, declined to comment to the Hartford Courant on whether he would veto the bill.

“I have been clear for weeks that a ban on the possession and sale of high capacity magazines is an important part of our effort to prevent gun violence,” Malloy said in a statement Monday. “Simply banning their sale moving forward would not be an effective solution.”

If the legislature passes the deal, it would become the third state to pass major gun reform since the December shooting in Newtown. In early January, New York passed sweeping gun reform that banned assault weapons, established universal background checks and limited magazines to seven rounds. Two weeks ago, however, New York Gov. Andrew Cuomo acknowledged the difficulty of enforcing a seven-round limit, saying that the laws would need to be reworked.

Only seven states and the District of Columbia have limits on the size or use of ammunition magazines.