check out the forum on Brian Enos' website:
www.brianenos.com.
Select the "Shooting Discussions" forum and then "Training Techniques" and look in there or do a search. They've had a long running discussion on the best snacks to eat during a long match.
I don't find it makes a lot of difference to me for a routine practice session, which for me is about 90 minutes and 250 rounds. If you're standing around all day at the range during a match or a class or a qualification, I do like to have a power bar of some variety mid-morning and mid-afternoon. Hydration is critical in hot weather. Caffeine is critical all the time.
Consuming protein is probably a good idea. Lots of carbs might make you sluggish, unless you are performing lots of physical exertion like hiking or something.