What's the correct edge between colors in current US Army/USMC armored vehicle camoflauge? Aside from using masks, how do you make hard edges with an airbrush having only a medium tip?

The edges on modern, three-color NATO camo should be soft. The best way to do this with an airbrush is to thin the pain to slightly thicker than water. I use Testors' Model Master (MM) Enamels exclusively. Paint the vehicle overall MM Medium Green. Next, adjust your airbrush pressure to 20 psi or lower, as low as you can and still get a flow of spray. You should be able to get the width of your airbrush down to about 1/8". Now use MM Black and spray on the black wavy areas. Next, switch to MM Field Drab and spray in the small areas of brownish color as per the camo scheme. If you do this method, there is no need to use a mask. Here is an example of how it turns out,
3-color NATO Camo on Panzer Howitzer 2000

I create soft edges buy cutting my camo pattern on a piece of paper and holding it 1/2" away from paint surface when spraying

I have used another method of soft edging.
Cut your template out of paper. To the back (Model) side tape or glue small portions of soda straw approximately 1/2 inch away from the edge. Tape or hold the straw against your model and spray straight on. The distance from the kit created by the straw and the setback of the straw allow for paint to overspray under the template.
You can use pretty much anything as the straw, just something to keep the template a uniform distance away from the kit.

I did this one and it worked ok. I don't know if I'll do it again....

Thanks to the replies! I was wondering because from some photos of the M1s in Janes AFV Recognition Guide Book (2001 edition), it looked like a hard edge.

Re: making soft edges, using an airbrush, I don't play too much with the pressure or make masks. Just paint and viola! soft edge (or what I think is a soft edge. #:-) )