rufous,
I beleive that Dave King wrote an excellent description of the prone shooting position taught at tactical shooting schools a while back. The basic position is very low, so low that it is hard on the neck and shoulders until you get used to it. You also get whacked on the collar-bone occassionally, you will know when that happens.
I have used the position in hunts with complete confidence and made some nice shots. I like to use my Cabela's fleece backpack for a rest, or a Harris or Underwood Shooting sticks. Always take the Underwoods, usually take the Harris's.
As for 400 yard field accuracy, I saw Steve Suttles shoot a 6 inch group at 400 - from sitting. You can imagine what he can do from prone. I am confident that guys on this forum can shoot five shots well inside three inches if the wind is not too ugly. Before leaving for a hunt this fall I cranked off five shots at 400 with my GA Precision "ROCK" that went 3 5/8" but three of them measure 0.459" outside to outside. It was quite windy and my buddies claim that the wind blew those shots into my group...