SPAS 12's of Jurassic Park
It should come as no surprise that many people who are familiar with the SPAS 12 had their first exposure to the shotgun from seeing Robert Muldoon carry it famously in Jurassic Park. Jurassic Park did a phenomenal job at prop selection and design, both finding and creating designs that have been permanently etched in the minds of fans since the film's release. From the Maxa Beam flashlights, to the iconic Jurassic Park Jeeps and Explorers, even down to the cryogenic can of Barbasol shaving cream, there's an entire herd of fans (they *do* move in herds) that have become enthralled with creating screen-accurate replicas of their favorite props from the movie. So, if you're wanting a truly screen-accurate SPAS you'll want to keep your eyes peeled for a few specific features.
Front Sight and Barrel
The SPAS 12's in Jurassic Park all feature a tiered/swooping front sight base attached to a 21.5" barrel . Cycling through the album to the right will show that it looks like all the SPAS 12's used in the film featured what I refer to as a "tiered" front sight blade, since the blade is almost on a platform as opposed to other variants where no platform exists.
Reveiver Markings
The only receivers that are visible throughout the movie seem to be the later style of FIE imported guns. They do not have the large SPAS 12 logo on the side as is seen on the older models of gun. You can also see the FIE importation markings on the left side of the receiver.
Trigger Group/Safety
The SPAS 12's in Jurassic Park all seem to have been equipped with un-recalled lever-style safeties. They were recalled and replaced with a traditional push-button safety after they were found to cause the gun to fire. (More information on that here.) It's also worth noting that the lever safety on the guns seems to magically flip back and forth during the "Clever Girl" scene, and is raised by Dr. Grant to fire at the incoming raptors while the safety is still on and unable to fire.
Folding Stock
The SPAS has two main designs of folding stocks, a latching folding stock that uses a spring-loaded pin system to latch itself securely on to the rear sight, and another variation that simply uses a "dog leg" protruding from it to secure the stock closed. There's also a variation of the "dog leg" stock that added in little ramps to make it easier to fold and deploy. The SPAS 12's in Jurassic Park all very clearly use the latching style folding stock as signified by the completely rounded area on the hinging part of the folding stock, and are why Muldoon has to make the specific movements he does during the "Clever girl" scene. They also do not appear to have a "keyring" style sling mount on the side of the hinge-pin for the stock as some later variants had, which was probably why the slings were mounted on the holes of the stock itself, allowing Muldoon to deploy the stock.
Accessories
The good news for you JP fans with a budget is not only that SPAS 12's with lever safeties are generally less expensive, but you also don't have to worry about finding two fairly rare accessories to be "screen accurate", the factory hook/arm rest, and the factory sling. Neither were used in Jurassic Park.
The slings in the movie are not Franchi factory slings. It is unsure what exact slings they were, but that is probably because they are very generic in appearance. They are also strangely clipped on to the holes of the folding stock. This could have been done for several reasons. One reason being that when using the top sling mount (the swivel drum mount on top of the pistol grip) the range of motion of the stock is encumbered, and Muldoon wouldn't be able to deploy the stock as he so famously does. Another reason is it's possible that, due to their known fragility, the top sling mount (swivel drum) was not used for fear of breaking.
It's a topic of contention, at least for a few Project members, as to if Muldoon's sling at the begininng of the film is dark green or black. The slings used by both Muldoon in the end of the movie are both certainly black.
Prop Gun
I have been able to find one image of a rubber prop SPAS 12 with a claim it was used in the filming of the movie. Note the obvious addition of the flashlight which made no appearance in the movie. It is possible that it was used in the Push Team sequence at the beginning of the film and simply out of sight or hard to see. Even this rubber model seems to have the tiered front sight and latching folding stock.
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(Edit 2016 - It looks just like the SPAS 12 leaning up against a truck in The Lost World, probably used there.)
Image found at proparchives.com