Having fired both a 357 mag Desert eagle, and a 50, I think the last thing I'd call them is a paper wieght. Like anything, a tool is only as good as the craftsman using it.
Now, back on topic, if I were building guns for a sci-fi flick (and what do you know, I have) I'm partial to using clamshells to help hide the profile of the actual firearm.
Battlestar Galactica used clamshells for the first couple of seasons, and I thought that was pretty cool.
I-Robot also used revolvers hidden inside shells, as well as a PPK for spooners back-up.
Judge Dredd, while being a crap film had some cool guns in it. Same with Pluto Nash. Some of those guns had a cool look IMHO.
Blade Runner hid it's revolver inside the action and magwell of a steyr SL, and it's grip in a custom build.
Constantine has a great custom shotgun all tricked out and hidden.
Aliens use of metal shells around a thompson was pretty damn cool. The 870 in the spas cage was nothing less than inspired.
The first Starship Troopers also did a fine job of hiding the real guns inside shells too. of course, it's the dreaded "bullpup" style though, which never bothered me.
I think the trick is keeping the guns from feeling like toys. The hero pulse rifles weighed in at 30 pounds or something, so they used lighter materials for starship troopers, and lighter base guns.
I've also had occasion to just add on custom pieces to older style weapons like derringers, webley's, and smith model 3's to create a certain look or feel to the firearms.
And when I'm working with directors and producers who don't know anything about guns, I try to guide them to the best choices, and make them feel like it was their idea all along, and happily give them all the credit if it saves me headaches and makes the actors look cool. I'm more concerned about the end product than anything else.
I've had some very good luck with directors and producers though.