Sounding concerns about terrorism, two Contra Costa supervisors are looking to ban the sale of a rifle they say can penetrate a refinery storage tank and ignite the toxic contents.

Supervisors John Gioia of Richmond and Gayle Uilkema of Lafayette have introduced an ordinance forbidding the sale of .50-caliber rifles in the county's unincorporated areas. The full board will consider the restrictions Tuesday.

Although there are two dealers in those areas, neither sells the .50-caliber rifles. However, the supervisors say they hope cities throughout the county would follow suit and adopt mirror ordinances.

``Ultimately, the public benefits from banning the transfer of these weapons,'' Gioia said. ``I feel comfortable that people throughout Contra Costa don't think these high-powered weapons should be sold.''

As Tuesday's vote nears, the National Rifle Association has Contra Costa County on its radar. Attorneys for the gun advocacy group have already drafted a lawsuit to challenge the ordinance.

Weighing up to 40 pounds and costing an average of $5,000, the rifle is not likely to emerge as the gun of choice for a terrorist or criminal, said Chuck Michel, the gun association's chief attorney. The gun has only been connected to one crime nationwide, he said.

The controversial rifle is primarily a military weapon, with skilled marksmen able to hit targets more than 1,000 yards away. A small community of aficionados also hold .50-caliber shooting competitions.

East Bay residents can buy shotguns at the Martinez Gun Club, which is in an unincorporated area. Pro shop owner Joe Calim said he supports the ordinance because .50-caliber weapons are too dangerous to be readily available.

Gun control advocates regard the impending Contra Costa lawsuit as a crucial test of whether the ban can withstand litigation. Long Beach passed a similar ordinance last year but later rescinded it. Los Angeles also approved restrictive legislation, though the city council is reconsidering the language.

Meanwhile, Assemblyman Paul Koretz, D-West Hollywood, has proposed legislation, AB 50, regulating the sale and possession of the weapon.

With a dearth of affected dealers and the county staring at a projected $82.4 million shortfall, this is hardly the time to play the guinea pig, said Kris Hunt, executive director of the Contra Costa Taxpayers Association.

``This is going to cost us money we don't have,'' she said. ``This has all the appearance of a very costly political gesture.''