Let's say you wanted to buy an Airsoft gun as a serious training tool. You wanted the closest to the real thing as possible. You want one that is going to hold up to training for years. You want the best of the best regardless of cost: money is no object.
Which one do you buy ?
Oh boy. Good question. Good, scary, maddeningly complex question.
In the airsoft world, we call just such a query "what am best gun?" There is no answer.
There are three realism factors in airsoft - Function, weight, and trademarks, in that order (usually). Unfortunately, here in the USA there are strict laws on the trademark front, and unless a company is licensed by the other company that they're making replicas of, no airsoft guns bearing realistic trademarks may be imported or sold in the united states.
A couple of companies
do license their trademarks. The problem? Most of the ones that do in the USA license them to companies that make crap guns.
Taurus, for example, licenses to CyberGun - Maker of those horrid transparent plastic things you buy at Walmart that break after two magazines full. Smith & Wesson licenses to KWC, who is a mixed bag. They make a lot of cheap junk and some truly horrid gas guns (the gas Sigma leaps to mind...) but also a few gems. IMI also licenses to them, and they make a truly kickass Desert Eagle. Western Arms (not Western Airsoft, by the way) licenses many 1911 clones as well as some Beretta stuff. Western Arms guns are top notch for detail and functioning realism, but at a cost - Since they're made in Japan (where metal guns aren't allowed to be sold by law) they're all plastic. They require a ton of maintenance and can't run off of green gas without taking damage - The most common fuel for gas guns in the 'states.
To my knowledge, H&K and Glock don't license to anybody in the USA. That's why the clones of their guns you get will be totally trademark free - Or have fake engravings. Not that it really matters.
For realism of function you'll want a gas gun, hands down. There are three kinds of airsoft pistol: Spring, electric, and gas. Spring guns have to be cocked each shot (usually by racking the slide) and are generally inferior plastic copies with minimal power and accuracy (though they can be had cheaply). "Electric blowback" pistols are novelties only. They run off of 4 AAA batteries (generally) and fire light BB's at dismal velocities. They also have a long trigger delay because each pull winds the slide back with a motor and the BB is fired when the slide slams forward, bringing the piston with it. Gas guns come in two forms - Non blowback and blowback. The difference is pretty clear - Gas blowback guns have working hammers or strikers and cycle their slides in exactly the same manner as a real semiautomatic pistol. Non blowback pistols work like the double action Wal Mart special BB pistols you're probably already familiar with, using a long double action trigger pull to raise and drop the hammer, slinging a BB out the barrel. Some have hammers that can be cocked for single action and cosmetic rackable slides.
On the weight front you'll want a metal slided gun at the least (some guns, like the HFC M190, are all metal). Here are some good choices, and the pros and cons of each:
KWA Glock series - KWA is the outside Japan export name of KSC. KWA guns are made in Taiwan in the same factories and with the same internal parts and designs of KSC guns. However, KWA export market Glocks come with metal slides as standard equipment whereas KSC guns do not. Note that in the USA the KWA Glocks will
not come with the Glock trigger safety (copyright dispute) and therefore have no mechanical safety whatsoever! You can order the trigger safety part from overseas for about a dollar and install it yourself, if you like. These guns are eerily reliable, incredibly easy to find parts and upgrades for, and fairly realistic in weight (with the metal slide) and function. For added fun you can get a G18c for full auto tomfoolery or the fictional G26c which is a G26 frame with a G18c full auto mechanism in it. You can run all KWA guns off of green gas which is a plus - R134a ("blue gas") is nearly impossible to buy stateside, and must be imported at tremendous cost.
Tokyo Marui guns are tough to find stateside but are generally detailed and well built. They're a Japanese maker like Western Arms, so their guns are plastic. Usually TM guns have a staggering array of parts for replacement and upgrade available, including metal bodies and slides. They're reliable and well finished, even for plastic, and
some of them (not all) can safely handle green gas. They're usually expensive and you have to import, though.
HFC Beretta series - HFC is another Taiwan based manufacturer who mostly makes knockoffs of Tokyo Marui guns, as well as a staggering collection of spring guns. Their M9 knockoff isn't actually a copy of any real gun - It has the underbarrel rail of the Vertec series but a curved backstrap like the original M92. It's true double/single action with a safety, but it has no decocker. The only way to decock the gun is to drop the magazine and pull the trigger (unlike a real firearm, airsoft pistols are safe to drop the hammer on a loaded chamber with, because the gas is actually in the magazine). However, their M190 (as they call it - see the trademark blurb above) is made entirely out of cold, unforgiving metal. It weighs a ton and feels quite real. It's engravings are either silly or practical (depending on who you ask) with BB gun-esque engravings identifying the gun as an airsoft pistol, warning that it's not a toy, and encouraging the user to read the owner's manual. It also comes in a nifty hard case, and takes green gas. A CO2 verion and a full auto version (!) are also available. HFC M190's are incredibly sturdy and reliable guns. I owned one for about two years, happily knocking the snot out of it, and then sold it to a friend who continued to do the same. Last match he dropped it on a rock about four times, lost in in a creek, found it again, and it still worked.
The other side of the Beretta coin is the KWA version of the M92, which
is a replica of the real M92 and features a working decocker. It can take green gas but has a plastic slide and body. Also of note is the KWA M93R2, which is the modified full-auto M92 variant as seen in Robocop (and probably a few other movies). It has good detail, working semi-, full-auto, and three shot burst. It's also got a plastic body and slide but takes green gas. It's clockwork shot counter mechanism can be finicky, though.
WE are a maker out of Taiwan that I am not entirely sure isn't related to HFC somehow. Their guns use similar materials and have similar workmanship while retaining the usual strategy - Knocking off Tokyo Marui designs. Their shtick at the moment is churning out version after version of 1911 doublestacker. Most of their guns are patterned off of tricked out 1911-based 'raceguns', some of their models simply sporting fancy grips, hammers, and triggers and others with custom half-cut slides, serrations, pointless shiny bits, and underbarrel rails. I have one of their "plain" guns (detailed in an above post) which seems every bit as tanklike as my old M190 as well as eerily accurate. WE guns can take green gas all day long, and few CO2 models are available. I'd stick with one of the less fancy models if I were you. Also, they're probably about 90% part compatable with Tokyo Marui guns for upgrades. They feature working thumb and grip safeties and strip down just like a real 1911. Even the three-pronged spring beneath the backstrap is the same as the real thing. WE guns sport no trademarks (even in their native Taiwan).
Western Arms guns are expensive, licensed, incredibly detailed, popular, and
expensive. They're from Japan, but have secured licenses from many makers like Beretta, Kimber, Colt, Wilson, and others to make fully trademarked, detail accurate guns. Mostly they make 1911 clones, but they also make Beretta 92's and 8000's. All of their guns are plastic and are made to run from lower pressured r134a, which must be imported unless you want to fiddle with modifying cans of air duster or A/C recharge. Parts are highly available, including metal bodies and slides for kitting your gun out to feel like the real thing at probably 150% of it's cost. I'm not a big fan of WA.
There are oodles of other gas guns out there, but these are the ones I have a lot of personal experience on. I'm pretty knowledgeable about this stuff, so if you have a question about a particular gun let me know and I'll try to answer it.
I recommend a KWA Glock, Beretta, or WE 1911, personally.