By Katty Kay BBC correspondent in Pennsylvania

Jason is taking part in military training in rural Pennsylvania. But he is a mercenary, not a government soldier, and this is America's latest boom industry.

Private armies are being increasingly used by the Pentagon

Once seen as shady, men like him are fast becoming mainstream.

Private companies like Northbridge Services do jobs governments can't or won't extend to.

The Pentagon has given $300bn of contracts to companies like these over the past decade alone.

"Private armies put the skills at a higher level," says Tom Patire, President of the International Training Commission.

"They also learn to use limited resources. Governmentally, whoever hires these people doesn't have to put up with all the costs.

"It's a flat fee, there are no medical expenses. There's no insurance. They hire them for a job. When the job's over, they let them go."

Overstretch

The industry grew from providing bodyguards for celebrities. Now it's more than that.

Mercenaries are training to do the kind of work the US Government can't afford to have its own soldiers doing. And in a couple of weeks some of them will be shipping out to places like Iraq and Afghanistan.

Will US soldiers like these one day be hired?

Are mercenaries the answer? For some jobs maybe. But there is still a huge resistance to privatising peacekeeping.

"The big risk is one of political accountability," says Michael Vickers, of the Centre for Strategic and Budgetary Assessments.

"There's a sense that even if they're operating under US or international command, the standard that one would expect of them for political accountability - if there are accidental deaths - would likely be higher that it would be for governmental troops."

When United Nations peacekeepers operate in places like Bosnia, they are bound to a national military code of justice and can be held responsible for their actions. Mercenaries are subject only to the laws of the marketplace.

Hiring muscle for money raises serious ethical questions.

But it's only a matter of time before the world's next humanitarian crisis demands troops and looks for them under a corporate flag.