How to use CAD and CNC machining to build an AR lower at home

By » Wed, November 23 2011

IMG 3004 How to use CAD and CNC machining to build an AR lower at home

Who doesn’t love the fact that all the tools, materials, equipment, plans, tutorials, guides, how-tos, and parts to build your own firearms at home is available online?

I’ve been meaning to try this for a while now (using the AR-15 lower receiver solid model from, but I wanted to do it using all open source software, and was having trouble finding something that could generate tool paths from the AR-15 model. PyCAM seemed to be the best bet, but whenever I tried it on very complex model it would very quickly use up all the memory (8GB of ram plus 8GB of swap) and bring the computer to a screeching halt.

I recently had a project at work where I needed to make a much simpler part, and not wanting to go back to using BobCAD under Windows, I gave HeeksCAD and PyCAM a try. It worked out pretty well, and in the process I discovered and fixed a memory leak in PyCAM

PyCAM is still horribly inefficient in its memory usage, but with the memory leak fixed and a new computer with 16GB of ram I was finally able to generate decent toolpaths for the AR-15 lower.

The equipment/software:

So now I’m all set to give this a try, but I can’t find the block of aluminum I had intended to use. But I did have a block of Delrin left over from an earlier project, and Delrin should be strong enough to handle a .22 cal upper. So I’m trying to make a delrin lower first for use with a .22 cal upper, and if that works I’ll order some aluminum and make another lower for use with a .223 upper.

The end result: the “gun” part of an AR-15.  Legally speaking, anyway.  This wouldn’t be the first plastic lower receiver ever made for AR-type rifles.  For a time, there was a company called Cavalry Arms that made polymer receivers for ARs; they went out of business for legal reasons, not financial, as their products were popular and had a good reputation.

sulawan 4726 tfb How to use CAD and CNC machining to build an AR lower at home

Delrin is probably not as durable as glass-filled nylon, but as stated, for an upper chambered in .22 long rifle, that’s not realistically an issue. You would want to use something harder like aluminum to machine your own receiver if you intended to build a rifle chambered in a larger caliber.

What’s the most interesting thing about this is that it showcases something a lot of people might not know: it’s legal to make your own firearms in the United States.  As many as you want, too, provided you can already legally own a firearm. And you can sell it, but it cannot be manufactured with the intent to sell it.

From the ATF:

With certain exceptions a firearm may be made by a non-licensee provided it is not for sale and the maker is not prohibited from possessing firearms. However, a person is prohibited from assembling a non-sporting semi-automatic rifle or non-sporting shotgun from imported parts. In addition, the making of an NFA firearm requires a tax payment and approval by ATF. An application to make a machine gun will not be approved unless documentation is submitted showing that the firearm is being made for a Federal or State agency.

[18 U.S.C. 922(o) and (r), 26 U.S.C. 5822, 27 CFR 478.39, 479.62 and 479.105]

The firearm does need to comply with other statues; it must have at least a 16″ barrel if it’s a rifle, or a barrel greater than 12″ but an overall rifle length of over 26″. And it can’t be fully-automatic, natch.

I guess the going thinking is that people who build guns in their own homes have only themselves to fault if something goes wrong.

rsrgp5907 450x101 How to use CAD and CNC machining to build an AR lower at home

The rest of the rifle, the “upper”, can be bought and transferred without a federal firearms licensee as the go-between, i.e. no background checks.  You can get them from all over, including Amazon: DPMS Upper 223 Rem 556NATO 16″, $463 w/free shipping.  This is the stress part of the gun; the lower just positions and retains the magazine, holds the trigger parts in place, and gives you something to hang a grip from.

You still need other parts, like the fire control group, furniture, the grip and the buttstock, and magazines. But all that isn’t legally a firearm, unlike the homemade part in question; the rest you can get that online and delivered straight to your door.

Of course, we’ll know the makr culture will have really come into its own when they have a CAD-controlled device that can cold-hammer-forge barrels.  Until then, some firearm parts will continue to be made by professionals.

Related: Popular:

Features, Know, Make