So I wouldn't go so far as to call the above "wrong" per se, more a misinformed perspective based on experiences with low-end consumer grade 3D printers probably not run particularly skillfully.
3D printing, as a technology, is really what you make of it and what you do with it. Unfortunately, as a technology, it became popular because of a few technological shortcuts. The result is that it earned a somewhat undeserved reputation for producing useless weak plastic trinkets. This is because the most popular, and easiest to print, material in the 3D printing market is called PLA. PLA is actually a pretty remarkable polymer in a lot of respects, but for this application it was chosen, not because it has good mechanical properties, but because it is easy to print. There have been major technological advances making PLA much better recently, however in general it still suffers from low toughness and creep under load. This all isn't helped because getting print settings right in order to maximize mechanical properties also involves some trial, error, and test equipment..... something many people can't/won't do, instead focusing on simple visual quality improvements at the expense of performance.
Just to illustrate the other end of the 3D printing spectrum, SpaceX's Dragon capsule uses 3D printed rocket engines, hypergolic fueled little monsters called the superdraco thrusters. Despite a lot of 3D printing naysayers arguing they were the cause of a recent explosion, it turns out not only were they not responsible, one of them actually survived the explosion which is remarkable full stop. (ref.
https://youtu.be/Fl3Jcczz5PY) I realize this is a bit of a divergence from the topic, but it really frosts my cake when people disparage 3D printing in general based on broad misconceptions of the technology. For example, a replacement trigger guard could likely be printed from stainless steel for approximately 35$. And such a trigger guard should easily exceed even the most ludicrous strength requirements for such a part. (rough estimate based on what I expect the size of the trigger guard to be based on 15 years working in the industry and not including dev. costs to achieve said design)
The point I'm trying to make here is that this is EXACTLY the right technology for this application. Getting someone with a CNC to design a part and then develop the gcode to make a single part would almost certainly be a pretty involved process. Meanwhile rapid-prototyping could crack out a working model pretty quickly. From there you can move to higher end print technologies, or if it really is a demanding application it can be CNC machined. Notable is that, even when I'm going to machine a part, I'll typically print test samples first because it is much faster and less expensive.
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What would be involved in 3D printing a replacement?
Well the starting point really wouldn't be all that different whether you want to print or CNC machine one. You need, if not the original part, everything that the original part has to interact with. This is because projects like this necessarily start with a digital model. 3D printers and CNC milling machines are really the same thing at their core, one just adds material with the other removes it. Hell, they even take the same machine code. This is critical though, you need to create a virtual version of the part one way or another.
From there, there are a few automated hand-wavey steps, and then you produce a prototype. If you're 3D printing, this is generally done in whatever cheap material the person with the machine has kicking around. Depending on how many guesses were involved in the trigger guard's design, the prototype might work great or it might need a couple revisions. Either way this cycle of design, test, repeat goes on until you're happy with the final result in a prototype material.
From there you get to your "production" material. This is generally a higher end material. I personally would probably go with 20% carbon fiber reinforced nylon. I use it a lot of things where strength matters. It also looks great, at least in my opinion. It has exceptional layer adhesion, great toughness, and great strength. Nylon alone is what Glock frames are made of. There are a LOT of different technologies out there though, and most of them are accessible to the consumer. Stainless steel is pretty inexpensive and accessible if you want strong. Brass is quite pretty, so is bronze or sterling silver. Titanium, aluminum, dozens of different resins...... really pretty much anything you can imagine. And, again, unlike some of the technologies Fabrisonic works with, this stuff is all consumer accessible for a production run of 1 unit.
So my advice to you is find a local friend or buddy who has said consumer grade printer and the ability to do a little 3D modeling. Have him crack out a design and iterate until you're happy with the form factor. Bonus points if this guy knows about whatever the final production print technology you want to use works like. Then take that to someone, or some company, who can print in higher end materials and get it manufactured in whatever material you want. I suggest this because, if you don't have the original part and have to supply the entire rifle and stock, doing it in person sure beats the hell out of having to ship the whole thing to someone. And you can be there to provide feedback in real time about how you like the design.
I hope all that is fair and made sense. When it comes to CNC machining of single parts, most job shops won't even give you the time of day. If you can find a retiree with machines and time to spare, maybe you'll luck out, but if the part is going to be a pain to make..... it seems like a low probability play. This is what 3D printing was made for, this is what 3D printing is good at. Take advantage of it.
Anyway I hope that helps/answers your questions. If you have any more, feel free to hit me with a PM. I'm glad to try and help.
