The answer is yes, a single direct hit from a paintball will shatter a 77mm glass filter. See the recent thread UV filter for paintball? for an explanation of why I wanted to do this test. So, here's what I did.
I mounted two 77mm filters (one threaded into the other) on a piece of deck board with handyman's helper, and used more handyman's helper to attach it to a tree. My son Trevor shot it from about 30 feet away with an Automeg 68 Classic with freak kit at an estimated 280 fps muzzle velocity, and the front filter was completely shattered with one slightly off-centre hit. The entire impact was within the filter's glass disc. The "behind" filter got a small amount of paint, but was otherwise unmarked. The broken filter was a generic 77mm sky filter labeled Access MC Japan - it looks pretty much like a contemporary Tiffen - or at least it used to. So there you go. I'm still not sure if I'll use my good B+W MRC UV filters on my 24-70L, 70-200L IS and 300/4L IS lenses (3 lenses, 3 bodies, no waiting) next time I venture into the paintball arena. I'm confident that the probability of getting splatter on the front element is higher than getting a direct hit, so I may use the filters anyway. I won't use double filters, as I want to keep the IQ as high as possible, but I think the flying droplets of paint are a sufficiently hostile environment to warrant using single filters.
I think you're mad not going for two filters based on your test. I don't have 2 filters to try it but I can't imagine there would be any drop in IQ worth mentioning.
Get some bright overalls too. When I last went the paintball place had some flourecent yellow vests workmen wear that they gave to two stags that were there on a couple of the games. Get one of them, unless there are also stags that are given them to wear, in which case that might be a very bad idea.
We use special paint for the winter here in the Great White North. It doesn't freeze, but it doesn't clean up as well as the warm weather stuff. The drop in IQ with two filters is highest with regards to flare; more air/glass interfaces = more flare. For me, the choice is between one filter and no filter. The question I must answer for myself is: is it better to risk breaking a B+W MRC to ensure that I get no splatter on my 24-70-200-300L front element, or to accept that I might get some stray splatter and risk that I take a hit on the front element ? I assume that the element is tough enough to resist breaking, and that's not an assumption that I plan to test in the near future.
