For the first time in almost 50 years, the U.S. Army wants to replace the standard rifle shouldered by hundreds of thousands of frontline troops around the world.

The service this week advertised its interest in a new weapon that would incorporate futuristic sights and other advances in rifle design and be able to handle improved ammunition.

The gun would potentially supplant the M4 carbine, a shorter-barrel version of the M16, the Army's main infantry weapon for decades.

Operations in Afghanistan—where troops often engage the enemy over long distances—have rekindled debate over the quality of the Army's standard-issue rifles and their reliability in dusty, primitive conditions. An Army report on a 2008 battle in Wanat, Afghanistan, cited soldier complaints about jamming and overheating M4s, in particular. Nine servicemen died in that fight.

Critics have also raised concerns about the range and lethality of the 5.56 mm cartridge of the M16/M4.