Not really sure why the Army Corps of Engineers is more strict on their background checks, finger printing, etc. Have done that, been there, and here now .. although have volunteered for the Fish & Wildlife Service, and didn't go thru the hoops we did for the US Army Corps of Engineers.
We enjoy volunteering for the Oregon State Parks, and went thru the whole background, finger printing, etc. thing, but since all that went to the State of Oregon that makes it much easier to volunteer. When we see a posting, we send an email, and talk with the Rangers. Not sure how often we have to go thru the background checks, etc., but so far have been ok.
One point to make though, if you are working/volunteering where access to the Government/Military Computers then that is an altogether more intense back ground, etc. check.