According to calorielab.com, a full on burger king whopper meal with large fries and a drink is going to come in right around that 2500 calories. If someone says that a soldier with low bodyfat to begin with can operate for a full day on 300 calories without drastically reduced performance, they are just plain ignorant, or worse, lying.

Running 15 minutes cross country is going to burn 900. You can see how any serious physical activity is going to use up those ration foods in a hurry.

Think about your average farm worker or construction laborer. When I worked construction it was like this:

Breakfast burrito at 5:30 AM with coffee and some sort of bread item.

8 AM: Powerbar

10:30 Roughly 2 lbs of something off the canteen truck. If it was the Korean truck then some big plate of some pasta oriental stuff, if it was the Mexican truck it would be one of their monster burritos.

Noonish: Another power bar

2 PM, another power bar

3PM, a stop at a fast food joint on the way home.

8 PM, another dinner.

And with that sort of diet, I was barely maintaining a weight of 220.

Thing is, you can do a two or three day mission on more limited food, and then there is the whole issue of pack space and weight. Powerbars and protein bars are a biggie, same with the trail mix stuff. You can get pretty big bags of tail mix at walmart for $5 per bag.

The meat snack beef jerkey type stuff can be a problem since it is usually unusually high in salt. That's why a lot of distance runners shy away from it. The stuff works well as a treat, and as an added ingredient in some sort of camp stew, but chowing down on it for the bulk of your calories is going to be a problem.