I was at Ft McCoy lately and saw a soldier with a tab that read Presidents 100 on his shoulder. I know this is a ranking in some marksmanship competition but I dont know the whole story. could somebody fill me in?

The Presidents Match is conducted every year at the National Matches held at Camp Perry, Ohio.  The top 100 shooters of the match are designated "The President's 100".  The match is open to all civilian and military shooters and is fired with the service rifle.

A little more..

The match is only done at camp perry once a year so it is a very difficult task to make the pres 100 and it is a very coveted award for high power service rifle shooters usually up to 1500 people shoot the match.

30 shot match, 10 off hand at 200 yds, 10 shot rapid fire string prone at 300 and 10 shots slow fire at 600.Max score 300-30x.

The year I went the winner shot 299 or 300 and the 100 th place was a 287-5x or so. I was out of it after the offhand stage and ended up in the mid 300th somewhere..