I did actually see a show once where the president of a company that adds armor and bullet proff glass to SUVs and such demo'ed the results by getting into a Suburban his company worked on and had people shoot at it with AK-47s from close range.

The truck was totalled, but nothing entered the cabin. I think the guys outside were more in danger of being hit by stuff flying off the truck than the guy inside.

With an XL2, the best you can get is to shoot 60i and shutter fast (lots of light) at 1/120. frame tightly on a target behind the glass so you can see the contrast of the bullet...did I mention lots of light?

you can then export the odd fields and the even fields separately using a deinterlacer and shuffle them together like playing cards. giving you a 50% slowmo...you'll want to fire a bunch to make sure at least one or two of them are caught hitting by the camera.

I would put the camera on the gun end of the room but offset some and frame zoomed in to compress the space...this will give the bullet the maximum time in frame. The lexan and towel are great ideas. I have a demo posted of this technique and an applescript that will sort image sequences from odds and evens in order (exported as image sequences from FCP in my testing).

link: http://www.yafiunderground.com/Video/slomo.mov