SCA: Society for Creative Anachronism (sca.org)
covers basically 600-1600. although earlier personas are accepted w/o to much grumbling. approx. 75,000 members worldwide.
the biggest battles are at Pennsic Wars in mid August each year just north of Pittsburgh, PA. (
http://www.pennsic.net/)
thing with most events though is they're semiprivate. there's a fee to cover site costs and you have to be in period costume (reasonable attempt).
there are most likely local groups near you you've never heard about.
Alan: if you're a legitimate target for a sling, you're already wearing armor. we make it pretty safe.