WASHINGTON - As the horrific details continue to emerge from the Connecticut massacre, lawmakers, advocacy groups and gun experts have begun weighing in on policy changes that they say could reduce the chances of another mass shooting.

But experts said it was far from clear that any proposal -- including extending background check requirements to purchasers in private transactions or a renewed ban on assault weapons and high-capacity ammunition magazines -- would have single-handedly prevented 20-year-old Adam Lanza from opening fire on defenseless pupils, teachers and administrators at Sandy Hook Elementary School.

What's more, some experts doubt that, despite a fresh round of interest by lawmakers, Congress would pass any reform measures, citing the muscular gun lobby and some polling data that indicates dwindling public support for tighter gun rules.

At the center of the gun control debate is the expired ban on assault weapons and high-capacity magazines, which was enacted in 1994. Some political analysts blame this law, which was heavily supported by Democrats, for the GOP takeover of Congress later that year. The rules, formally called the Federal Assault Weapons Ban, expired a decade later when Congress did not extend them.

Sen. Dianne Feinstein, D-Calif., said Sunday on NBC's "Meet the Press" that she planned to introduce legislation to outlaw assault weapons on the first day of the new session of Congress in January. Feinstein said her legislation would ban the possession or sale of assault weapons and "big clips, drums or strips of more than 10 bullets." The purpose of the bill, she said, is to get "weapons of war off the streets."

In the Connecticut school shooting, Lanza reportedly used a Bushmaster .223 semiautomatic rifle as his primary weapon.

At least two handguns also were found at the school. Authorities have said the guns belonged to his mother, Nancy, who Adam Lanza killed at the home they shared before going to the school.

It's unclear whether the specific model of semiautomatic rifle Lanza reportedly used would have been forbidden under the expired assault weapons ban. The Bushmaster .223 "wasn't banned by name" in the 1994 assault weapons ban, said Daniel Webster, director of the Johns Hopkins Bloomberg School of Public Health's Center for Gun Policy and Research, and a supporter of stricter gun laws.

Jonathan Lowy, director of the Legal Action Project at the Brady Center to Prevent Gun Violence, a Washington-based advocacy group, noted that "virtually all" of the suspects in mass shootings in recent years have used high-capacity magazines or an assault weapon.

"I really believe that this most recent shooting has the potential to be a seminal galvanizing event in the history of this issue," Lowy said.

But any proposal to curtail access to guns would be met with ferocious opposition from gun rights supporters.

In the wake of Friday's attack, Gun Owners of America (GOA), a Springfield, Va.-based lobbying group, and other gun rights advocates called for the repeal of the federal Gun Free Schools Zones Act, saying that because the law forbids most adults working in schools to carry firearms on the premises, they can't protect themselves and their students from attackers.

"In addition to the gunman, blood is on the hands of members of Congress and the Connecticut legislators who voted to ban guns from all schools in Connecticut [and most other states]," wrote Larry Pratt, executive director of GOA. "They are the ones who made it illegal to defend oneself with a gun in a school when that is the only effective way of resisting a gunman."

Other lawmakers are calling for hearings and investigative panels. In appearances on "Fox News Sunday," Sen. Dick Durbin, D-Ill., said he planned to hold a hearing in January on gun rights, and outgoing Sen. Joe Lieberman, I-Conn., called for a national commission to review gun laws, mental health services and violent video games.