So I'm seeing more and more stories about people having AR lowers made using 3D printers.

As someone who is very familiar with 3D printing, I knew it would only be  a matter of time before people would start making them. The lower cost of 3d Printers, along with higher tensile (stronger) polymers, means that making polymer gun parts with 3D printing makes sense.

The feds are getting very worried that the technology is allowing legal gun owners to bypass their asinine gun laws.

I recently had some AR lowers 3D printed using ABS. I built up a unit with a .22 conversion, and it was flawless. I don't think I'd ever fire anything bigger than .22 with it, although I analyzed the properties of the extrusion grade ABS, and it is definitely strong enough to handle .223 forces, at least in semi-auto mode.

I wouldn't do a polymer upper for .223, because the heat could cause the ABS to soften, causing the bolt to get mis-aligned with the barrel or stick to the upper, causing FTF and FTE faults.

I did a bunch of tests using a .22 conversion, and it is a blast! A 16" unit built using a .625 pencil barrel is super light (less than 5 pounds unloaded).

watch my polymer AR lower video here

Just use ultem or ppsf

So, what would legally keep someone from building (milling?) a lower using ABS, for example, instead of aluminum? It would (should) be considered at the most an 80% lower and provided you follow the published guidelines I would think you would be GTG.

Of course the ATF is worried, the 80% lower rules are allowed because their isn't much interest in joe shmoe milling anything often enough to matter. Until now anyway.

no law against building your own firearm from scratch.

Lemme see if I can get the video to work...

Hope these images show up...

lower 1

lower 2

One was run with .007" resolution, and the other run with .010" resolution. Both from ABS.

Machine was a Stratasys Dimension 3D printer running black ABS.

LPKs dropped in fine with no mods needed. Both cycled fine in both .22 and .223.

Actual gun was super lightweight.

Real quick - I'm using a Fortus 400mc Large machine (production T-class) and have a pretty good design now for an ABS lower.

Here's the kicker - just because one can print an AR lower from plastic doesn't mean it's going to work. You have to keep in mind that the AR architecture that everyone is familiar with was designed to be made in ALUMINIM. Specifically:
   - Mil-Spec AR15 Lower - 7075-T6 Aluminum (Tensile ~75,000 psi, Yield ~21,000 psi)
   - (Fortus) ABS M30 (Tensile ~5,200 psi ––> over 14 times weaker than 7075-T6)
   - (Fortus) ULTEM 9085 - strongest material available for the Fortus machines (Tensile ~10,400 psi ––> over 7 times weaker than 7075-T6)

What does this mean? Well, through trial and error (or a good FEA program) you'll need to beef up the current AR lower design (available for download via grabcad.com or 3dcontentcentral.com) in a few specific areas to get close to handling field abuse. (Here's a few hints - look at the buffer tube tower, takedown lugs, and bolt catch hinge points. Holes also need to be adjusted to allow for 3D printer tolerances).

I've got a bull-pup design printing right now as I speak that's based around a dedicated 22cal upper...but I'm concurently working on a Rev 2 that incorporates a built in buffer-tube for use with a 9mm conversion and buffer system similar to the 4"L one from Gunsmoke.

3D printing technology is growing by leaps and bounds. I was recently at a training session hosted by Stratasys at one of their Eden Prarie, Minnasota, facilities and they are also a job-shop in addition ot manufacturing the Fortus, Dimension, uPrint, and Mojo machines. They've got at least 50 to 100 printers running all the time on jobs! (Majority of them 400mc and 900mc production machines).

There are some out there that are trying to put together a 100% 3D printed firearm - and to them I say good luck. Typical cartridge chamber pressures for pistols and rifles range from 8,000 psig to more than 50,000 psig. Create a plastic that can handle this kind of pressure and you'll never have to worry about money again! Until that time comes though firearms will ALWAYS require metal pressure retaining components - so you gun grabbing morons out there can stop worrying about a "metal-detector invisible" firearm. Not going to happen. Perhaps you should put less stock in Hollywood (ala In The Line of Fire) and instead take some basic Materials Engineering courses at your local university. (http://news.yahoo.com/you-don-t-bring-a-3d-printer-to-a-gun-fight––––yet.html)

Lastly - Why print _most_ of a gun? Because printing knives and hammers is boring. I can't tell you how cool it is to go nuts designing anything you can think of in Solidworks, follow with a little STL file processing, press PRINT, and a few hours later you've got a model you can hold in your hand. ("Few" is a loose term as it takes around 57hrs to print a complete bullpup "kit", about 12hrs for the ABS-optimized lower)

How much did your "printed" lower actually weigh?

The average aluminum stripped lower is only 8-9 ounces.  A few less if you skeletonize it.