Here's the real info, trust me, I've been with the RWVA Project Appleseed 2 years now, am an Instructor and am RWVA Senior Staff.
We say "run what ya brung". .22 to 8mm is best, though don't worry about a .204 or .17 caliber, they're fine, too.
.50 BMG rifles won't be real welcome, but if you came with one, we'd stick a loaner in your hand.
SKS? Yup, Fred himself has shot Rifleman with an SKS. He likes them, too.
AK? Yup, them too. Try to make sure you've got an accurate one. Romanian AK's (SAR or WASR) are pretty good, you'll be fighting a short sight radius, but, it can be done with an AK.
AR, M1, M1A/M14, FAL, G3/CETME, Dragunov? Been done, they work.
About ANY military bolt gun works, make sure you've got enough strippers for it.
.22's have got to be the most common thing. Most of the Course of Fire is at 25 meters (83 feet), and a .22 works well. Make sure to have 4 10 round mags, if you can, as you'll want them.
Scope or Iron sights is acceptable, and if your choice is between a rear sight on the barrel (especially on a .22) and a scope, go with the scope (AK's and SKS's adjust with the front, BTW). You're going to NEED to make sight adjustments. Tech Sights on any rifle is nice, especially on a .22.
Ruger 10/22's that have been mildly altered (Liberty Training Rifles) are extremely popular.
Now, let's assume you don't have an adequate rifle, and are planning on going to an Appleseed in the area. ASK one of RWVA folks if we can arrange a loaner. I personally have 6 10/22's that get used, plus a couple other .22's. If I'm working a shoot, they are available, first come, first served, reservations taken. Other Instructors loan their rifles out too. Dwarven1, crak and Martlet are the other RWVA folks on this forum.