“So how strong does an AR15 style lower receiver have to be. Couldn’t it be made of plastic or even wood, so long as the holes all line up?” [captain_dave, post 21]
“People have printed lowers out of plastic with 3D printers but they are for demonstration purposes and not considered strong or durable enough for actual use with standard cartridges.” [fireman15, post 25]
“... the most well known 3-D printed gun, the Liberator 3D-printable pistol....” [fireman15, post 65]
The notion that 3D printing will fully supplant more traditional materials and methods in gun manufacture is nonsensical.
Wood is unsuitable for lower receivers: not stiff enough, not stable enough in a dimensional sense.
Polymer suffers from the same limitations that rule out wood, but (in proper formulations only) has become viable thanks to innovative design, and incorporation of composite materials and stiffening inserts. Makers of successful polymer-frame arms guard their plastic formulation jealously; the prospective 3D-print home workshop gun builder is not going succeed by substituting bulk plastic pellets from Hobby Lobby.
Plain unalloyed aluminum is not used in gun parts manufacture - doesn’t wear very well. If one can obtain suitable alloy, it still isn’t a simple matter of proper accuracy and precision in machining. The major aluminum parts of AR-15-style arms are in part forged, not machined from blanks. And following forging and machining, they are selectively heat-treated.
3D printing cannot overcome any of these limitations. But there are greater constraints 3D printing will never overcome: working parts like barrels, bolts, firing pins, hammers, sears, triggers must be carburized (case-hardened) or heat-treated and tempered. Unhardened, untempered parts may work in theory, but at best will deliver only a few shots before wearing out of spec. And steel is the only metal strong enough to stand the pressures of militarily effective cartridges. Forum members valuing their own safety - and that of loved ones - are urged to avoid 3D-printed guns purporting to handle such.
Springs are the toughest limitation of all: they can be fashioned of only a limited number of materials, and they must be tempered after forming. No 3D printing process will ever create a spring.
Gunmakers are not blind to the advantages of built-up parts (of which 3D printing is only the most recent iteration), but they have labored for generations to improve the techniques needed (sintering and MIM are two); many gun owners have had problems with parts made thus, and are not at all willing to agree that they are suitable replacements for more traditional methods.
Even if theoretically perfect, suitably precise parts of adequate temper and hardness could be fashioned by some hitherto-undiscovered variation of 3D printing or other buildup technique, they cannot be just slapped together to become a functioning gun. Hand fitting and adjustment are always required: safe and functional tolerance ranges are simply too small.
The self-absorbed revolutionaries who are so convinced digital changes everything need to stick to what they know.
Of course you are correct in your observations about firearm production using digital production methods. A knowledgeable and skilled person can easily produce a firearm more durable, safer, more effective, and more quickly using hand tools and off the shelf materials from any hardware store.
But I am not so sure that you not missing the point of what those who are involved in this effort are really trying to accomplish. The point for the most point is not to produce cheaper and superior firearms. The point is to freak out and demoralize those who hope to restrict law abiding citizens access to firearms. I think that the point is to say fine you might be able to make it impossible to purchase a firearm but we will just use our ingenuity to help those who need or want a means of self defense to get one anyway.