That would be my main concern. I’ve made prints on my resin printer that came out slightly smaller than the dimensions in the STL file. I’ve also had prints warp slightly during UV curing, although that has happened mostly with large thin pieces. It will undoubtedly take some trial and error to get a part to come out to the exact dimensions you want. The company making the mouthpieces has of course had lots of time and experience, that you would have to go through yourself.
Other options didn’t really exist then, except paying $$$ for a Prusa
Things have also advanced since the 2013 Liberator design. You’re printing the receiver for the most part in these designs (=the part that makes it legally a firearm), the barrel and such is metal in most cases, and usually a common design like Glock 17/19 or AR. Rebels have been using them in Myanmar.
I have a parts kit with a torch cut receiver that I could get around building some day, but I’m not going to try to convince the library to make it. It’s a vz. 61 Škorpion (or “Klobb” if you were young and had a Nintendo 64 in the 90s). You can also weld the receiver together but that’s a lot more out of my skill set.
“Bullet tracing” is a not very reliable science except in TV shows. At best it’s another method to strengthen a case with other evidence alongside it.
That’s the same parts kit I have. I wondered where the “Klobb” came from. I decided to go high end and bought a steel Czech receiver. It is amazing quality. The receiver is quite complex with various slots, grooves and blind holes. I’m a hobby machinist and the finish is so smooth everywhere on this thing I can’t figure out how they machined it. Looks like it was molded. Not a tool mark anywhere.
Here is what they look like from a different company: