"At that point, I felt I made a commitment to these guys that I would make them for them," he said, "so I had to go raise money, try to find a way to get these made as quickly as possible."
Greenwood quit his job as a successful home remodeler, and asked friends and family to invest in his new company. He set up a Web site and charges around $500 per Cooper Sling.
"Put yourself in Kyle's position," Hartmann said. "I mean, do you tell them, 'I know this is a safer seat for you. I know this will help you do your job much better. But this is only for my friend.'"
Greenwood has sold more than 4,000 Cooper Slings to soldiers in the United States, Iraq and Afghanistan. They are sold on the Internet and mailed directly to the war zones. Some are purchased by field commanders and some by civilians through an adopt-a-gunner program, while other soldiers have bought Cooper Slings with their own money. Canadian soldiers have bought them, too.
Greenwood started receiving unsolicited letters from soldiers in the field, like one from Sgt. Daniel Rodriguez after a truck smashed into his Humvee. "Your Cooper Sling kept me from being ejected out of the vehicle," Greenwood said, reading it aloud.
He read another letter from a soldier in the Texas National Guard on the front lines in Iraq: "The Cooper Sling saved my life. And my wife and soon-to-be five kids appreciate it."
The Pentagon has so far resisted any efforts to make the Cooper Sling standard issue.
In fact, Greenwood's mission to get a Cooper Sling to every gunner was dealt a blow last month when the Army posted a warning urging soldiers not to use it.
The warning said testing had showed that "it did not prevent the gunner from being ejected out of the gunner's hatch and would actually prevent rapid entry into the vehicle crew compartment during a roll-over drill."
Greenwood was especially surprised to hear this warning because he had received letters from Army commanders thanking him for saving their soldiers' lives.
The Pentagon would not comment except to reiterate its position that testing had showed it to be unsafe and to add that it "directs that any of these items installed on Army vehicles be removed immediately and replaced with an authorized seat."
Hartmann has since come back safe from Iraq, perhaps the best testament to his buddy's invention.
"Worst-case scenario for me, it turns out to be a nonsuccessful business venture," Greenwood said. "This isn't really a sacrifice for me. The ones who are making a sacrifice are guys over there defending our country."
Pentagon commanders have taken heat for not always providing the best equipment for soldiers -- whether that be inadequate armor for a soldier's vehicle or for his body.
Regarding the Cooper Sling, the Army has agreed to take another look and will have it tested at an Army-approved facility at the end of this month.