A letter from a Central Florida soldier has raised new concerns about a military rifle used by thousands of American troops, according to a Local 6 investigation.

U.S. troops have been using a shorter version of the M-16 assault rifle called the M-4 in Afghanistan. Experts said the weapon has a light modular design and the military considers it a great weapon.

However, in March 2002, Natik, the Army's main laboratory for developing weapons, found 15 percent of U.S. troops surveyed reported the M-4 jammed.

Also, 20 percent reported double feeding -- which is when two rounds of ammunition go in at the same time and cause a jam , the report said.

"Now, a local soldier's disturbing e-mail home suggests the M-4 is still failing our troops under fire," Local 6's Holfeld said.

Former U.S. Army Sgt. Hassan Vertley is making his brother's e-mail public to expose alleged M-4 miscues.

"Yesterday afternoon while on patrol we came under enemy fire while walking on the side of a mountain," Vertley read from his brother's e-mail.

Specialist Wayneshway Vertley said his weapon jammed in a firefight that he was convinced cost his sergeant's life, Holfeld reported.

Vertley's M-4 story raises questions over the weapon's reliability in the field, Holfeld said.

More than 50 percent of the soldiers in the Natik survey reported problems cleaning the weapon because of the Afghanistan sand.

Defense Week reported that the Army's typical solution to be: Apply more elbow grease, the Local 6 report said.

Former Marine Sgt. Jose Ferrer said the M-4 is a reliable weapon but any machine has problems as it ages.

"So, it does jam?" Holfeld asked.

"Absolutely," Ferrer said.

"Why?" Holfeld said. 'Because it overheats or what?

"No, no, no," Ferrer said. "Like I said, any machine can jam when you are in a combat environment. You might not have time to clean it and that's why you have to train people on how to clear malfunctions when they occur."

Rep. Ric Keller is working with Local 6 to examine the M-4's reliability in the field. It is the same weapon used in Iraq, according to the report.

The Defense Department has always stressed maintenance is key to keeping any weapon working.

Watch Local 6 News for more on this story.