The gun-making thing is - while "true" - a misdirection; it's not a whole gun but the lower receiver for an AR-15, which would normally be a folded/stamped U-shaped piece of steel. You can also make such a receiver by beating a shovel flat, bending it up and doing a little cutting and the shovel is much more durable. The plastic printed receiver lasted for 6 rounds before breaking and it requires a proper bolt, barrel, etc in order to make an actual gun. Printing receivers is a weird oddity of US gun laws where the registration (such as it is(n't)) of a firearm somehow attaches to the receiver rather than the barrel or breech or anything actually functional, whereas barrels etc are freely available and uncontrolled.
And of course if you have machine tools, you can make a real gun. Apparently quite legal in the USA somehow.
As to cameras, for sure. You can already download plans to print out a holga-like camera that just needs a lens added. I fully intend to use the 3D printers at my local library to make repairs to my Jobo should that ever become necessary. I'm considering printing a 6x12 back...
Certainly you can print in metal; it works by laser-sintering layers of metal powder. I've seen a regeneratively cooled liquid rocket combustion chamber printed that way, with a design significantly more-complex (the internal cooling channels) than could be traditionally manufactured by casting or machining. And the Chinese have recently printed a 3m titanium wing spar. 3D printing of metals is serious business because you can make structures with internal voids while retaining strength; if you get it right then the result is lighter and stronger than any other manufacturing process.