It is possible to use 3d printers to create a form of firearm.
That said - they are not what I would consider effective. In order to start getting into the more advanced capabilities of 3d printing - you are going to have to spend some money and have a pretty good setup at home. The most appropriate systems to use for creating firearms are, new, as much as it would cost to set yourself up with the capability to manufacture firearms, conventionally.
Common 3d printers use thermoplastic extrusion, with some using a form of UV and laser sintering of powdered plastic. While these machines are quite versatile in what they can accomplish, and one can theoretically construct rudimentary small-caliber firearms out of them; you need to pump it up to Direct Laser Metal Sintering or a derivative using an actively scanned electron gun (think vacuum tube TV).
These systems allow you to fuse metal powder layers within an inert environment to form a solid structure. There are a few limitations with the device, but the key one applying to firearms is that they create a very 'rough' surface that often requires post-processing to refine. This is fine for most structural applications - but is unsuitable for the fabrication of standard and especially rifled barrels. Proper chamber spacing and dimensions often have tolerances that are not within the accuracy tolerance of current generation 3d printers.
You might get something that would -function-... but not likely very well. The closest you could do is to form a solid cylinder that you then machine in a classic manner to form a barrel, chaber, etc.
And that is the most critical process of firearm manufacture.
And it is still cheaper to invest in the tools to do it the 'old fashioned way' than it is to invest in attempts to do it with metal-capable printing solutions (easily $100,000 just to get started in that line of printers... and you're looking at $300,000-500,000 to get into anything worth trying to use for barrel/chamber manufacture). An enterprising individual could probably develop, largely, their own similar solution - but even then - there are key parts required that are not widely available as salvage. Industry liquidation auctions with such things present are likely to have people at them that know their value - so you're not likely to get a steal, either.
The big thing that 3d printing changes is that a number of things that once required several machining processes can be condensed into one step that is easily ported from CAD to prototype. It has also led to a new service industry - machine shops that used to do custom orders for parts can now field low-volume requests almost effortlessly compared to 15 years ago while working with machines that are still out of the budget range of the garage machine shop.