Now, as Congress considers spending millions more on the guns, this exclusive arrangement is being criticized as a bad deal for American forces as well as taxpayers, according to interviews and research conducted by the Associated Press.

"What we have is a fat contractor in Colt who's gotten very rich off our wars in Iraq and Afghanistan," said Sen. Tom Coburn (R-Okla.).

At about $1,500 apiece, the M4 is overpriced, according to Coburn. It jams too often in sandy environments such as Iraq, he adds, and requires far more maintenance than more durable carbines.

U.S. military officials don't agree. They call the M4 an excellent carbine.

There's not a weapon out there that's significantly better than the M4," said Col. Robert Radcliffe

Coburn is the M4's harshest and most vocal critic

his concern is shared by others, who point to the "SCAR," made by Belgian armorer FN Herstal, and the HK416, produced by Germany's Heckler & Koch,

Both weapons cost about the same as the M4