CNC machining an AR-15 lower update and slight detour

In the comments of my last post, Grego pointed out where there are CAD files for a multi-part AR-15 lower where all the parts fit nicely in the working area of a Taig mill. It also doesn’t require any special jigs or the crazy long, thin drill bit for making the hole for the bolt catch pin. Since I wouldn’t need to make any jigs or buy any drill bits to finish that version, I figured I’d try it before continuing on with the normal one-piece lower. I went straight to aluminium since I had a slab the right thickness.

Starting to mill the left half of the lower out of a slab of aluminium.

After the end of the roughing pass, where you can see some pretty crappy edges because I was making cuts that were too aggressive for the Taig.

After a couple of contouring passes to give it the right shape.

After the finishing pass.

Now, use a band saw to cut off the excess aluminium block, flip the part over, and start roughing the other side.

The four completed parts of the lower and the bolts that hold them together.

The finish is funky on the inside of one of the halves because, due to software issues, I was having trouble getting accurate Z positioning.  I managed to fix the problem for the other parts.

The assembled lower.

With the fire control parts installed.

And, finally, the completed firearm.

That’s everything except the bolt catch. Since the bolt catch is held on by a press in roll pin, I’m going to leave that off until the lower is painted, rather than have to press it out again to do the painting.  My plan is to treat with Alodine and then spray on Duracoat. I think that should work out pretty well, and it is easier than anodizing.

I modified the design a bit from the original. My version is thinner (though still not as thin as a standard AR lower), and instead of using a custom extended selector switch, I cut a recess so that a standard selector switch would fit.

Now that that’s done, I’m going to try to finish the one piece Delrin lower. But first, I think I need to make some sort of enclosure for the mill to keep the chips from getting all over the place. When working on parts that big, it makes a hell of a mess. Normally, when I use the mill for work, I’m just working with small Delrin parts, making cutouts in aluminium sheets, or etching small printing circuit boards.

19 Responses to “CNC machining an AR-15 lower update and slight detour”

  1. Jason,

    Great job!

    What model is your mill, and how long did it take you to complete the lower?

    I’ve seen a bolt-together lower somewhere else on the web, just can’t lay hands on a link right now.

  2. Grego says:

    It is great to see that cad file used by someone else. I would be interested in seeing more about what you changed (and if you are interested, i’d love to chat offline, I hope sebastian could pass along my email from the comments to you). I’ve put thousands of rounds through the original that you based the model on, and it still works well (doubly so on how it clamps the upper in hard)

  3. mike says:

    Was running the mill loud? Wondering if it’s something that can be done in an apartment without upsetting the neighbors.

  4. Roberta X says:


    …But I do have to point out that a “bit” goes in a brace, a drill goes in a drill press or drillmotor. “Drill bit” is a modern coinage that abuses the language, right down there with “flammable.” (And cutters, of course, go in a mill.)

  5. MAJ Mike says:

    Is it necessary to register the lower receiver with the BATF? Are there any requirements from the Feds for numbering and tracking?

    Just asking because we all know that commercial lower receivers are controlled serial-numbered items. Guess you could make any lower reciever — MP5, HK91, AK74, etc.

    Cool garage project.

    • persiflage says:

      Hmmm…registration. Did you just make it for yourself, or do you plan to “sell” it or “transfer” it to someone else? Might make a big difference. The PTB want it to be identified by type, serial number, make & model (among other info) for a transfer with a 4473 record.

      • MAJ Mike says:

        Don’t get me wrong. I don’t advocate registration, but I’d like to give the BATF as little excuse as possible for a mid-night visit. I suspect that someone at the Ministry of Truth cruises gun blogs for “business”.

        • Kristopher says:

          If you are making a rifle, shotgun, or pistol for your own personal use, you don’t need to deal with any federal regs except 922r.

          ( synopsis of 922r: if it is a semi-auto with three “assault weapon” features, it has to have a majority of US made parts in it ).

    • Diomed says:

      As long as the firearm is not subject to the NFA and it is not made for resale, there are no special federal rules. You can make as many guns for yourself as you want, so long as you follow the other federal laws that apply to guns (922(r) is the main one). State laws may vary.

  6. PT says:


    If you can redesign the mag well to be thicker, there is a market for .45acp lowers. A standard lower won’t fit the adapter block.

  7. alcade says:

    That’s pretty cool! I’d like to do something like that… my workplace has an anodizing line and I think that lower would look really cool black or bronze.

    Is it 6061? T5 or T6?

    • Jason says:

      6061. At least half is. The other half was from some scrap aluminium I had lying around. It seemed to look and machine like 6061, but I don’t really know what it is.

  8. mobo says:

    Most excellent, sir!

    I have a question relating to pistol builds that I have never been able to get a definitive answer to, and I’m hoping you might know the answer. Is it legal to use an unmodified, standard M4 receiver extension on a pistol build, as long as the actual stock is removed? I understand that a constructive intent charge could result if you are in possession of a separate stock, but what if this were not the case, and instead you only have the buffer tube on an assembled pistol?

    • D2k says:

      I think the pistol buffer tubes exist because constructive intent gets used by the ATF in some really scary ways and it’s better to play it safe.

  9. TIM says:

    I was just wondering if using a cnc to make the parts why go with the bolt together design.Couldnt you just mill it as a one piece.

  10. Some Sock Puppet says:

    It is legal to make your own firearms as long as you don’t sell them? What about a handgun in aan area that requires a license to purchase? I would think you’d need the license anyway, but if AR lowers aren’t restricted in this way are handguns?


    • Some Sock Puppet says:

      I’m extremely interested in this idea. Does anyone have any idea on this? it may be simple to someone else, but I have trouble with non-literal statements.

      Can you craft your own handgun in a state that requires you to have a license to own or carry?

  11. Made to perfection, What RPM spindle on the mill


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