The firearm props available here fall under several different categories. How do you know what you should use? I've compiled a short checklist of considerations here, but feel free to email or call me if you want to discuss specifics:
Does the gun need to be fired? If so, do you need the noise, smoke, and visceral actor feedback of a blank-firing gun, or would it better to use a non-firing, gas blowback, non-gun, or other prop and add the effects in post-production? If not, do you want something that still has moving parts, looks good in a close-up, or is safe for physical stunts?
What's your budget? Obvious, but people sometimes forget that when designing their big shootout. Besides cost of weapon props, you may also need to consider ammunition expenses, and the cost or difficulty of showing the damage they cause. This may include blood effects, destruction of sets or scenery, stunt rigging, etc.
What will show? Do you need close-ups, or is this just set dressing for the extras? Is the color something that will be visible given your backgrounds and lighting?
Safety! Does someone need to put a gun in their mouth and pull the trigger? Is someone being shot at point blank? These things need to be thought through carefully and done with extreme caution.
How much abuse is it going to take? If the gun gets kicked across the room, it needs to be very sturdy or cheaply and easily replaced. Any time someone falls with a gun, is disarmed, beaten, etc there is a potential for damage to props (as well as actors!). Is the prop you want to use something you could afford to replace if neccessary?
How much ammo is going to be used? Depending on your genre, you may not care if you have the whole 'unlimited ammo' thing going on in your film, but someone in the audience will. Some guns hold more ammo than others: if the character is going to need to walk into a room and shoot ten people without reloading, don't give them a revolver.
Is it appropriate to the time and location of the setting? If you're doing a period piece, you may need to research when certain weapons were manufactured, and where they were available.
Is volume an issue? Semi-automatic blank firing guns require full loads, so there's enough force to push the slide back, eject the casing, and chamber a new round. These are very loud, especially in enclosed sets. Some use 9mm blanks, many use 8mm blanks, and some even use little .22 blanks. Revolvers, on the other hand, can use crimp blanks, which can be in any load you desire- from full volume to fairly quiet, in whatever their calibre: 9mm (common), 8mm (rare), or .22 (common for smaller revolvers). A step down from crimp blanks would be the caps in the Marushin blowback replicas; these small caps (often 5 to 7mm and very short) fit inside re-useable brass cartridges, and give off smoke and light with less noise than a real blank.
How important is reliability? While any type of gun can be made well or made poorly, revolvers have the advantage in that if a revolver misfires once, you can just keep pulling the trigger and it'll move on to the next round. This may not help if you need that one take on film, or if it really has to be the first shot, but sometimes on stage especially this can be a showsaver. If a semi-automatic misfires, it can jam, and at best you'll have to rack the slide and shake out the jammed casing before continuing... at worst, it'll have to be fixed after the show. Fully automatic weapons, such as the Marushin uzi, Mac-10 and M4, are especially prone to jamming- but then some (like the M93r modelgun and the Jackal blank firing machine pistol) have been flawless for me so far.
What kind of permits are needed, and how do the choices effect insurance, permits, film location availability, etc? You may not always have the option of firing loud blanks.
What look are you going for? Props can lend a lot to the atmosphere of a production- brand new and slick, old and battered and primitive. . . plus your lighting designer may want to have a say in things. The addition of flashlights, laser pointers, bayonettes, silencers, or other accessories can be important to the design of a particular scene- the lighting, sound, staging, and filming.
Does the weapon chosen fit their hands well enough for them to use it? If the gun is too large for them and they're supposed to reload it, take the safety off, etc. they may not be able to do this. If it's too small, or they are wearing bulky gloves or other costume elements they may have difficulty operating the prop.
Is the actor left handed? Some firearms work for either hand, but many don't. Also a consideration if you're doing the two-handgun style.
How strong is the actor's grip? This hadn't occurred to me until one year I was working on a production where an actress couldn't pull the trigger on a prop revolver they wanted her to use (not double action, anyway; without pulling the hammer back before each shot). This is partly a matter of strength, partly a matter of fit, and really only an issue with certain double-action revolvers.
Is the actor trained and comfortable with what they're using? You'd be surprised how big a difference even just a half hour of training can make.
Does the actor need ear or eye protection if using blanks? Will this be acceptable for the shots you want?
Do you need to give your actor something to work off? Some actors can pretend just fine when using a fake gun, but others need something to cue off, or to react to, or just to convince them they don't really need to be making machine gun sounds themselves. Weight is also an issue here - some actors can pretend a gun has a real weight to it, but others may need the solidity of a heavy prop to convey that to the camera. On the other hand, if a gun is too heavy and the actor isn't as strong as the character they're portraying, it may show.
What would this character have access to, and the training to use? If they are police or military, they may well have a standard issue of weapon for the given time period and setting. Otherwise period, cost, nation of origin, availability, and legality of a given firearm should all be considered.
Transportation; is this something the actor can carry and/or conceal if needed? Ok, if you want to be comical and have someone pull a full size shotgun out of their pants, go for it, but be aware of how silly it is. Tucking a handgun in your pants is generally a very bad idea, unless you have a holster there.
What is the job at hand? If the character is sent to guard a narrow, short hallway against zombie attack, a shotgun is going to be a much better choice than a sniper rifle; but if they're on a rooftop trying to save some hostages from someone in a building across the street, that shotgun isn't going to do them much good. As Robert DiNiro tells Sean Bean in "Ronin", the smart man doesn't play favorites. It's a toolbox; you use whatever tools you need to get the job done.
Accessorize your war[drobe]: Scopes, laser pointers, bayonettes, bipods, flashlights, extra ammo, holsters, cases, magazines. . .
Style! Ok, for some of your audience a gun is a gun is a gun, but you've got another shot at character development here. What he or she is packing can say something about their character.