AmaLite Inc., a maker of a civilian version of the military's M-16 rifle, came up with what it thought would be a way to capitalize on the expiration of the 10-year-old ban on semiautomatic assault weapons.
For an advance payment, the company offered on its Web site to set aside kits with all the parts for an authentic "pre-ban" rifle, complete with its military accessories, and to ship a fully assembled version to customers after the ban expires, which it is set to do Monday.
Armalite, though, has received very few orders. Mark Westrom, the company's president, said that demand was weak because the ban already had enough exceptions for gun enthusiasts to continue buying new guns that are essentially semiautomatic assault weapons with just a few minor adaptations. Existing assault weapons also remained in circulation despite the ban, which was passed by Congress and signed by President Bill Clinton in 1994 over the strenuous opposition of the National Rifle Association.
"All along, these firearms have been available," said Gary Mehalik, a spokesman for the National Shooting Sports Foundation, the gun industry trade association. "It has always been possible to go into gun stores and buy a pre-ban firearm or a firearm cosmetically altered to comply with the provisions of the ban."
The ready availability of these powerful weapons underscores important questions often overlooked in the political debate over whether the ban should be renewed, say both gun control advocates and members of the gun industry. How effective has the ban been, and what will happen if, as seems likely, it is not renewed?
The effectiveness is fiercely disputed and statistically hard to determine. There are different definitions of semiautomatic assault weapons, and there is no good national database to measure crimes committed by type of gun.
One study, conducted for the Brady Center to Prevent Gun Violence by two former officials of the Bureau of Alcohol, Tobacco and Firearms, found a 66 percent drop in the use of assault weapons in crimes after the ban was enacted compared with the five years before it went into effect.
The study, released this year, also concluded that had the law not been passed, approximately 66,000 more assault weapons would have been traced to crimes since 1994.
"What the study shows is that even with all its loopholes and weaknesses, the ban has substantially reduced the use of assault weapons by criminals," said Dennis A. Henigan, legal director of the Brady Center.
But another study, commissioned by the Justice Department, found only a small decline in crimes committed with semiautomatic assault weapons and said that it had been offset by a steady increase in crimes committed by other guns equipped with large-capacity magazines.
These are magazines that hold more than 10 bullets. They are a key feature of assault weapons, and the 1994 law prohibited their further manufacture in the United States. But the law permitted the continued purchase and sale of existing large-capacity magazines and imported ones, along with the estimated 1.5 million assault weapons already in the country.
Further complicating efforts to measure the impact is the way the law was written. It prohibited 19 kinds of semiautomatic firearms, including Uzi's, AK-47's and AR-15's, the civilian version of the M-16 rifle. It also barred guns equipped with two or more accessories that appear useful for the military or for criminals, but that have little recreational use. Among these are pistol grips for faster firing, flash suppressors, a folding stock and rifling to help attach a silencer.
To comply, a number of manufacturers simply changed the name of their guns or took away a few accessories, said Kristen Rand, legislative director for the Violence Policy Center, a gun control group. For example, Intratec, which made the banned TEC-9 pistol, changed a few features and came out with a gun called the AB-10, which stood for "After Ban."
Similarly, the Bushmaster XM-15 used by the Washington-area snipers to murder 10 people in 2002 was a copycat version of the banned Colt AR-15, Ms. Rand and others said.
The Violence Policy Center estimates that more than one million of what it calls "post-ban" assault weapons have been manufactured in the United States since 1994.
Wayne LaPierre, the executive vice president of the N.R.A., has repeatedly said that there is no reason to renew the ban because the guns that were banned are really no different from those that were not. "They simply lost an ad hoc beauty contest" when some of their features were disallowed, he wrote in a guest column in USA Today on Wednesday.
But that argument could also be a reason for voting for a more restrictive ban, gun control advocates say.
One statistic not in dispute is that the weapons are disproportionately involved in the killing of police officers. A Violence Policy Center study found that of the 211 police officers killed in the line of duty from 1998 through 2001, 41 were killed with an assault weapon, many of them post-ban models.
"Are we so foolish as not to realize why bad guys use these fast-firing, high-capacity guns to shoot police?" asked William Bratton, the police chief of Los Angeles who was in Washington on Wednesday to lobby for an extension of the ban.
Don Davis, the owner of Don's Guns in Indianapolis, the largest independent gun dealer in the nation, called the ban a reasonable idea, because assault weapons are made for combat, not hunting.
"But the law obviously didn't work - it was watered down so bad," Mr. Davis said. "People want them," he said, and with many companies continuing to make or import some version of the guns, "they have continued to sell like crazy."
Mr. Davis has in stock 150 large-capacity magazine clips for AK-47 rifles, and a number of clips for Glock semiautomatic pistols that can hold 30 rounds - well beyond the number turning them into assault weapons under the legal definition. "I've never been out of them, just like I've never been out of AK-47's," he said.
Brandon Hiott, who works at Heavy Metal Armory in Bradenton, Fla., said many gun enthusiasts looked forward to the ban's expiration, partly because they will be able to buy guns with more accessories.
"When's the last time you ever saw someone use a bayonet on a rifle to commit a crime?" Mr. Hiott asked. "The people who buy these guns legally aren't committing crimes."