The Sci-fi Weapon Room.
My main collecting addicition is for Sci-fi weapons from various movies and T.V series such as Aliens, Predator, Serenity and many others.
Now, if you like me can't afford to purchase screen used each and everytime the only other real option is to either scratch build one yourself or modify something!
Most weapons used in Sci-fi or fantasy movies are either custom made by prop artists or modified by using regular pistols, rifles for example. Those that are modified can make it easier to make your own hero prop replica, but sometimes trying to dissect how the weapon was made in the first can take sometime.
As I am investigating this alot, I thought it would be good to share this information so I will keep listing weapons below that have been made by modifying something else.
If I have missed anything drop me a line and let me know!
Aliens (1986)
Weapons
Armourers that made the weapons for this movie based all of the marines weapons on actual real firearms.
Pulse Rifle M41-A - Made from 3 real weapons
Mainbody is a Thompson M1A1 submachine gun (Replica builders purchase an airsoft replica to chop)

Franchi Spas-12 shotgun cage and pump grip (Replica builders purchase an airsoft replica to chop)

Remington M870 shotgun receiver and barrel (Replica builders purchase an airsoft replica to chop)

Custom made metal shrouds (Resin shrouds are available from various places and if you are lucky you can pick up metal reproductions)
Building a hero, metal Pulse Rifle can run you up a bill just short of $3000 by the time you have finished. Alternatively you could go with a mid-level option which would consist of a G&P kit which includes a shroud, electonics etc and then just purchase a Thompson replica. This would set you back around $1000.

Proposed replica of the pulse rifle by Hollywood Collectibles.
There are also many resin replicas on the market of varying quality, some are suitable for display, others for conventions etc but basically you pays your money and you makes your choice! One of the best all resin kits out there has to be from the Spat Cave, comes in kit form with a choice of working counter etc. Good value for money.
Spatcave's Spulse Rifle
I will be building a pulse rifle as part of my future projects, which will be listed separately soon.
HK VP-70 - Standard issue sidearm of the USCM.
Heckler & Koch VP-70
A numer of resin replicas exist of this sidearm, usually all cast as one piece but for those of you wanting a proper replica there are still a few airsoft replicas floating around.
Starwars
Blasters
Han Solo ESB DL-44
Mainbody is a "broomhandle" mauser pistol (Both resin and metal replicas are still readily available)
Custom machined flash muzzle (Resin and machined metal versions are often available)
Custom made scope bracket
2x Piston halves from a Revell V-8 engine model, these fit onto the opposite side from the scope
M19 or M32 brass azimuth telescope (Originals are expensive and getting more difficult to locate, so a resin scope if your best option.)
Picture of a Master Replicas Blaster
Luke Skywalker ESB DL-44 - Not much difference between Luke's blaster and Han's blaster, only subtle differences.
Mainbody is a "broomhandle" mauser pistol (Both resin and metal replicas are still readily available)
Custom machined flash muzzle (Resin and machined metal versions are often available)
Custom made scope bracket
Distributer retainter from a Revell V-8 engine model, this fits just above the trigger
M19 or M32 brass azimuth telescope (Originals are expensive and getting more difficult to locate, so a resin scope if your best option.)
Underworld (2003)
Selene's Signature Pistols - Seen alot during the movie and were also featured on the movie posters.
2x Modified Beretta M92FS fitted with compensators (Monsters In Motion supply these as a kit)

Selene's Back Up Pistols - In addition to the Berettas, Selene also used two of these in the film.
Two tone walther P99 fitted with BA-5 laser sights under the barrel (The laser sight is a pain to locate!)
Blade (1998)
Firearms - Beside Blade's glaive, darts and silver spikes he also used two firearms based on real world weapons.
Sidearm
Mainbody is a Mac-10 with a custom compensator. (The Mac-10 can still be picked up as an airsoft, metal replica and the custom parts are available from Spatcave.)

Blade II (2002)
Sidearm
Mainbody is a HK USP9mm
Custom forend module (I don't know of anyone who produces this part yet, if you do let me know!)
Laserpoint
Minimag solitaire model (Available from any good hardware store.)

Picture of Minimag solitaire