Is it permitted to go paintballing?
As far as I understand it, the three major Judaic objections would be:
A.) Putting yourself at risk of injury. B.) Putting others at risk of injury. C.) Developing wrong character traits within yourself by practicing shooting at your buddies. (Or similarly, trying to emulate the wrong types of people.)
(Any other objections that anyone can think of?)
As for A: the individual has the right to choose to increase his personal risk by some degree (but not to the point of "clear danger"; the risk must still be considered "a risk normal people take" [shomer pesaim Hashem].) I'd heard that from Rabbi Benny Lau quoting R' Shlomo Zalman Auerbach, I believe based on a Noda Bihuda.
B: R' Moshe Feinstein was asked regarding a professional athlete; doing his job meant putting both himself and the other players at some (small) additional risk. R' Moshe allows it, as the Talmud talks about people who got jobs doing somewhat dangerous things like climbing tall trees. R' Moshe says that the other players are cognizant of the risk, so that's not a problem. He is concerned that if the sport involves some danger to the crowd attending the game, they don't fully understand the risk, so that would be prohibited.
Although both A and B concerned someone's job, not their recreational activity.
As for C: The Noda Bihuda famously frowned upon game hunting (as a pasttime) -- even if you kill the animal fairly quickly and painlessly, why are you trying to be like Esav? It's hard to argue exactly where to draw this makes-a-moral-impression line; I know of one contemporary rosh yeshiva who even vetoed laser tag as an activity because it involved shooting people.
In sum, on all three issues, I think different rabbis may have different views on different situations. (So go ask your rabbi.)
One final point: If the paintballers serve in (or plan on serving in) law enforcement or the armed forces, an argument could certainly be made that this is part of training for that significant (and Torah-condoned) activity. But again, ask your rabbi.
I asked about this issue years back. My posek was OK with the idea of laser tag ("it's just a game"), but felt that paintballing was a midos problem (even if the other person is mochel, it is bad midos to put a bruise in another person").
Paintballing has less chance of injury then bowling and fishing. As for the manners, when your playing a game, just say to each other that at this and this range, don't shoot at all. And if a mistake happens, its just once per a group which isn't so bad, and definitely not life changing. I don't see how one mistake can change your midos, even IF it happened.
I recently asked Rav Hershel Schachter this question. The Rosh Yeshiva responded to me with a long detailed discussion on this topic. In conclusion he held that it prohibited under the issur of Chavalah. (See end of Perek HaChovel).
Rabbi Yosef Viener of Monsey also holds it prohibited.
Rav Tzvi Berkowitz, one of the maggidei shiur in Ner Yisrael and a world class Talmid Chachom holds it is Assur.