Still, paintball players must continue to unscrew their guns’ compressed-gas cylinders for recharge during play and at the end of the day. The valve design that proved deadly in the Contois case can loosen on the side that keeps gas under pressure in the cylinder. A player can then unwittingly release the valve, turning a charged cylinder into a dangerous rocket.

The problem is compounded by young paintballers’ frequent modification of carbon-dioxide-powered guns, in which a part called an antisiphon device is added to prevent jamming and increase the weapons’ performance. One such modification had been made to the gun whose cylinder killed Ms. Contois.

Rather than carbon dioxide, many guns now use compressed air. Because an antisiphon device has no role to play in these guns, they do not inspire modification, though their cylinders too can cause accidents.

While manufacturers have designed better valves since Ms. Contois’s death, by all accounts hundreds of thousands of the older ones are still used by paintballers. “There’s no way to reach them, because we don’t know who they are,” said J. J. Brookshire, chairman of the paintball subcommittee of ASTM International, a group that sets voluntary standards for a wide variety of products and services.

Mr. Brookshire said that the industry had improved consumer information about this and other safety issues and that technicians at the centers where paintballers must go to refill cylinders are trained to spot loose valves or those of old design. Further, standard 20-ounce carbon-dioxide cylinders must be refurbished or replaced every five years, a timetable that will eventually eliminate the old valves.

Image Mark Contois and his wife, Colette, who was killed in an accident at a paintball park.

Yet any canister under pressure can be dangerous. “This is certainly the most dangerous hazard” for paintballers, said Scott J. Wolfson, a spokesman for the Consumer Product Safety Commission. After accidents that were caused by a faulty valve mechanism different from the one in Ms. Contois’s death, Brass Eagle Inc. of Bentonville, Ark., recalled 243,000 of its paintball guns. Those guns, safety commission officials said, had been responsible for at least 73 accidental gas-cartridge ejections, in which seven people were injured.