US special operations forces have received a small number of new assault rifles optimised for the type of cave-complex fighting experienced in Afghanistan.

The weapons are based on the M-4/M-4A1 carbine variants of the M-16 assault rifle family, but fire the 7.62 x 39mm Soviet-designed cartridge and magazines used in the AK-47 assault rifle.

Knight's Armament Company of Vero Beach, Florida, delivered the first six rifles, called the SR-47 (Stoner Rifle-47), to the US Special Operations Command (USSOCOM) in January. The company received a contract for an initial six weapons in late October 2001.

David Lutz, vice president of military marketing for Knight's Armament Company, said: "For the last couple of years there was a requirement in USSOCOM for an addition to their [M-4 series carbine] SOPMOD [Special Operations Peculiar Modification] kit that they called a 'Special Purpose Receiver' (SPR).

"Originally they called it the SPR V1 for 'variant one', and it was to be a drop-in 7.62 x 39mm receiver replacement."

According to Lutz, government laboratory efforts to satisfy the SPR requirement were further complicated by "user input" from the USSOCOM specifying that special forces did not want to use the "straight box" M-16 magazines but instead wanted to utilize "battlefield pick-up" AK-47 magazines taken from opponents.

"That was a dilemma because the AK-47 magazine won't go well in a straight chute dimensional magazine - it just won't happen," he said.

He added: "But actually this program was kind of on a back-burner until US special operations guys were going into these complex of tunnels that were so deep, expansive and target-rich that they couldn't take enough loaded M-16 magazines. So they wanted a weapon that had all the muscle memory of an M-4 - safety, grip, everything that's familiar to the soldier or the SEAL - but capable of using battlefield pick-up magazines."

Instead of a drop-in receiver addition to the SOPMOD kit, design changes mandated a completely new weapon with resulting change in terminology from Special-Purpose Receiver to Special-Purpose Rifle (SPR).

"To enable the use of battlefield pick-up magazines, we had to make the upper and lower receiver 0.25in longer. You couldn't take an M-4 receiver and even machine it out to take the AK-47 magazine because it was too short. That also meant that the bolt carrier had to be made longer and the firing pin had to be made longer.

"So you started losing what some people would have liked to have in terms of optimal interchangeability of parts; that's just part of the trade-off to fire the different cartridges," Lutz said.

He highlighted the advantages of the 7.62mm size round for close-quarter battle (CQB) operations. Noting that many of the world's counter-terrorist organizations have evolved from 9mm to 5.56mm ammunition over the last decade, he highlighted the larger 7.62mm ammunition for the ability to package heavier, slower bullets that could provide greater contributions in CQB scenarios.

In addition to the extended upper and lower receivers, another challenging design effort in the SR-47 involved getting the M-4/M-16 magazine catch to externally function like the M-16 magazine but work with a curved AK-47 magazine. In practice, US soldiers use gravity to 'drop' their empty M-16 magazines. The SR-47 design requires the introduction of an internal magazine ejector to push the empty AK-47 magazine from the bottom of the weapon. Additional design features include the introduction of a free-floated match grade barrel.

"This particular 7.62 x 39mm is probably the most accurate 7.62 x 39mm in the world because it's got a really fine free-floated barrel," Lutz added.

"And, of course, it has the rail system so all of the other SOPMOD accessories off the M-4s are compatible.

"There's also a possibility, although they haven't let the contract yet, that there could be another variant that we'd call the SR-74. That could be used if our special operations guys go to a country that has the 'newer' 5.45mm former Soviet weapons. Then they would also have the same ability to pick up magazines."

All six of the SR-47s were delivered with sound suppressors, which Lutz described as "essential" in tunnel operations because of the weapon report.

"We don't know how the six did," he said. "We don't know if they are ever going to order one more. We don't know if we're going to get the second phase, which is to develop the 5.45mm version. But this is probably the hottest weapon that's out there right now."

Lutz concluded: "The SR-47 is a great gun because of the three technologies that it marries: the basic Stoner gun design; the AK-47 series cartridge and magazine; and the modular weapon concept."