I'm going to be making a computer game with a "western" setting and was wondering if it would be illegal to use guns from that era, like the popular Colt Single Action or the Schofield Model 3, and other weapons of course. I tried to look into it, but didn't find anything.
This article (warning: TVTropes) discusses several reasons why you might want to avoid this, as the name of the gun may be trademark, but the unique likeness of the gun may not be. It's done with other products (Rock Star games for example, uses this with Guns and Cars, and Pokemon games do this with map locations) to avoid any unfortunate implications of players use of the products or story elements (For example, there are plenty of car manufactures that would like it if Rock Star would not let their cars run over hookers in San Andres... which is difficult to control.).
Most of the time, in this case, the names are going to sound like the actual product or make a pun off of them (Rock Star games loves it's dirty puns, most often featuring sexual or scatological humor... or both). Others will just make cheeky references to the real product, and they can get pretty obvious if you know your stuff. For example, instead of featuring the "Colt Single Action" or "Schofield Model 3", try "Bolt Single Action" and "Shoefield Model B" which when read out loud in English fast enough sound like the real world equivilent, but with the serial numbers filed off (figuratively speaking... don't literally do that, it's a good way to get your self into legal trouble unrelated to copyright infringment).
I don't see a problem.
Colt may be a brand name that's still in use, but that's being used nominatively. Trade Marks protect abuse of a name, not the correct use. You may use Colt to name real Colts, not a cheap Chinese knock-off. In this case, it's an in-game virtual gun, but it still represents the original.
You might get into problems if you name your entire game something like 'Colt Single Action simulator 1848'. At that point you're using Colt as if it's a trade mark for the game, not the gun.