My opinions wind up being mixed. I've seen both good and bad LARPs and SCA events even if it's not really my cup of tea. I'm a huge nerd, but this kind of thing tends to bring arrogance and politics up to an entirely new level. Some SCA cantons and the various rankings and such get taken WAY too seriously. When it comes to fantasy LARPs and stuff the problem is that it can be hard to get the right mix of things going on, you wind up with a lot of costume discrimination, power trips based on fantasy ranks, and of course people just acting so outrageously nerdy that even other nerds occasionally have to step back and say "WTF".
See, these kinds of pictures, and versions of this kind of thing that show up on TV or in movies, tend to avoid the whole "fat marshmellow nerds in capes" aspect of things (and this is coming from a nerd who is truly obese nowadays). The relatively normal guys and gals, in fair shape, with cool costumes, are only part of it.
That said there would be absolutely *zero* problem in setting up a Warhammer 40k LARP. Most LARPS and similar events are intentionally set up at camps or in sections of parks or whatever which aren't heavily travelled. A local group called "Fantasy Quest" tended to rent a local 4H club. More high tech games, such as Cyberpunk and Vampire, oftentimes involve renting a ballroom, or someone involved donating their home or property. Things like paintball arenas and the like can also be used. In a lot of cases the guys organizing the events will sell tickets, and the tickets are used to rent the space and/or purchase dividers and the like and paint/decorate them to set up in ball rooms or whatever.
When it comes to gun combat, that's stupidly easy, and people doing Cyberpunk, Shadowrun, and Vampire-type LARPS have been doing it for ages. Typically I've noticed it being done with squirt guns (which come in crazy configurations) or occasionally paintball weapons. The idea is that you generally don't want to make a huge mess out of people's costumes or the environment. What's more, despite how it might LOOK at times, LARP combat that actually matters typically involves a referee to verify hits, and weapon effects, especially as seeing in a fantasy environment some characters will cast spells (oftentimes handled by tossing bean bags, or just by making a declaration to the rep), and while skill can play a role in combat, in a lot of settings that have "role play" as opposed to being purely historical, things like immunities, defensive spells, and similar things can come into play.
For the most part Warhammer 40k would probably be one of the easiest settings to do from a LARP perspective given that guys like Space Marines and the like are supposed to be comparatively rare. Reinactment uniforms are fairly obtainable, and big in certain paintball events to begin with. To say be part of The Imperial Guard all you need is something that looks like it could be a uniform (and it can vary greatly) and a gun for example, a Dark Cultist can just wear a Halloween robe or cape. Things like Powered Armor and the like are comparatively difficult to do, but you already see people making things like that for cons and such. One balancing factor in LARPs is that not everyone is uber, and gets to live out a personal power fantasy because you generally need to be able to have a costume for whatever your going to play. Not everyone can make or afford to have someone make them say chain mail or something that looks like plate, so thus not everyone gets to be knights and such. In comparison someone say picking up cheap vinyl or Nagahyde to stick onto a vest or jacket with some shiny "studs", or just wearing a leather jacket, or whatever, is comparatively easy and what a lot of people do, and usually wind up in comparatively low ranks. Wizards in fantasy events tend to be comparatively common (and surprisingly fairly weak) because robes and cloaks are relatively easy especially around Halloween. The same logic would of course apply to any kind of 40k event. Of course this tends to lead to the point about "costume discrimination" the guys that get to be powerful in a kingdom are the ones who have a lot of time or money to put into it. Someone who can say afford to drop $3000.00 to buy a set of chain mail, and/or spend the time learning how to fence, could wind up as say a Knight Or Baron within a couple of events. In comparison some dude who "RPs" well (or maybe just likes to hang out) but can only afford a shoddy costume (not being talented enough to
make one) could literally show up for years and not be able to do or have a say on anything. Of course there is also politics (ie your friends with The King, or one of the Canton organizers) and similar things. A lot of people very much wind up just helping to make the "scene" for others, especially in very historic settings where say a duke doesn't even have the remote possibility of one day getting nicked by the wrong beanbag/spell.
As I said, not my cup of tea, but I have known some people who were very, very, outrageously into this kind of stuff for a while. I thought about doing it, but decided not to, both from a financial perspective, and the fact that it seemed like it would be a giant headache waiting to happen. I don't much care for politics, and sleeping out in the woods while occasionally running around with boffer weapons and throwing beanbags around (or listen to nerds in costumes give speeches about how great they and their kingdom are, while otherwise walking around a Ren Faire) was something I figured wouldn't hold long term appeal. I showed up for a few SCA events and things and my reaction was kind of "meh".
