I think the airsoft alot of people are familar with is the toy crap. The "real" stuff is very nice, and shoots about as good as a paintball gun. The "real" stuff is the Gas Block Back pistols (GBBs, semi or full auto, slide cycles like a real one on every shot), and the Automatic Electric Guns (AEGs, select fire) which are usually subguns/rifles. The "springers" that you cock everytime you fire, and the little tiny toy full autos at guns shows are crappy toy trash. A stock AEG like MP5 model shoots about 280 fps. (and you can upgrade them easily/cheaply to shoot higher) A stock GBB pistol shoots around that fps too.
The mags on these guns are the actual magazines. You reload by swapping them out just like the real thing. The gas guns have a small refillable cartridge inside the mag body for the gas (refillable with gas from a can via a nipple on the bottom) and they also contain the actual BBs (25 rounds in my M9s case, slightly too many but hey...) So during play the slide actually locks back after the last round is fired, you drop the mag with the realistically placed mag release, slam in a fresh one, hit the slide release and you're ready to go. With the AEGs you don't actually cock the gun with the charging handle. You just slap in a new mag and it's ready to fire (only downside to them...).
Me and about 8 guys in the area including my younger brother (who has a Tokyo Mauri MP5 A5 AEG) use airsoft instead of paintball because of the extra realism. Holding a realistic gun, and doing realistic reloads and stuff is major fun, and is good practice IMO. We all carry 4-6 extra mags, use real milsurp LBVs, wear full BDUs, etc when playing. I'm sure it's a "OMG they're terrorists" panic phone call to the police waiting to happen, lol...
For playing "simulations" or war games or whatever you wanna call it, the AEGs are the ticket. The gas pistols would be decent indoors, but let me tell you are you outdistanced, and outgunned outdoors in the woods (as I suppose you would be in real life with a pistol vs a subgun/rifle).
I currently only own a Gas block back KSC Beretta M9. I got it since it was darn similar to my Taurus PT92 AF. And it is, it functions just like the real thing, mag reloads are accomplished exactly like the real thing, it is GREAT training IMO. The trigger setup is virtually identical and it feels the same, the safety works the same (the M9 is of course slightly different in this area from a Taurus, but the Airsoft M9 has a functioning decocker, full safety etc just like real). It has the same 3 dot sights, the mag release, and the slide release are in the same spots. If that isn't great practice to fake combat with a "toy" gun that close to your real one than I don't know what the hell is. There is no other way short of simunitions to get that kind of practice, shooting at real living, thinking opponents...
Of course when skirmishing with the guys I don't even use it, several of the other's have multiple AEGs so I just bum one of them for now. (I am too broke to spend $350 on one right now, I can another real AK for cheaper, but I can't shoot anyone with the real one without some consequences, lol) As I said you are outgunned trying to use a GBB pistol against AEGs.
About the airsoft being converted to fire real bullets, that is bull****. First off, the out barrels on these things is there for looks, it would not fire a bullet. Second off, on the AEGs they do not have a functioning bolt, would not cycle or load a road, and even if they did they would not fire it (no firing pin). The gas guns are very similar to a real pistol, but they do not have a firing pin, real mags do not quite fit in them, and there is not way in hell the gas assembly would fire off a live cartridge. (BTW, my KSC M9 is Taiwanese I believe). There are no currently made airsoft guns that wold be realistically converted to really fire. IFFFF you somehow did manage it would be explode on the first shot.
I will take some scans of my airsoft M9 vs my Taurus...