AR15 Two Part Lower Assembly

This is a brief overview of how my two-part printed Lower Receiver is assembled into a fully functional Lower Receiver.

Dvorak is a confused kitty.

You’re going to do what now?

 

I’m going to assemble an AR-15 Lower Receiver from parts. The lower Receiver is the part of the AR(Armalite Rifle)-15 civilian firearm that contains the fire control group, the magazine well, and attaches the stock to the Upper receiver which holds the bolt and the barrel.  On its own, the Lower Receiver is hardly recognizable as a firearm.  One would be more likely to confuse an upper receiver by itself for a firearm than a lower.  But the Lower Receiver is the part on which the serial number is stamped, and of all the distinct components of the complete rifle, this part is legally considered to be ‘the gun’ even without all the other parts.  Thus, a firearm transfer is required to just this part from anywhere outside my state(legally, you’re buying a firearm from out-of-state). But one is legally allowed to make one of these for personal use(not with intent to sell) without needing a firearm manufacturer’s license from the BATFE.  It is less expensive to print my own than to buy one.

These are the parts I will be using.

not printed.

Parts which will make a stripped Lower Receiver a fully-functioning lower.

After printing both parts of my two-part lower Receiver, I must remove the support material which I added to the models to facilitate printing. This support material was added by me in the model itself, so the model was sliced and printed without automatically-generated support material.  It was only necessary in a few specific areas, and milling material out of every qualifying cavity is very time-consuming.  And unnecessary with my modeled support.

Beards is a silly name for them.

A view of the model showing the support. And the beards for printing to prevent warp.

The support material can be broken away with a screwdriver.

The support material can be broken away with a screwdriver.

A pair of pliers can pull the support material out easily.

A pair of pliers can pull the support material out easily.

The support material is removed.

The support material is fully removed.

I pulled the beards off before taking any pictures.

What once was on the part as support material.

After the support  material is removed, the magazine well is tested for fit.  The printed model has artifacts from printing, and interior spaces seem to be slightly undersized after printing.

Once magazines fit properly in the well, the magazine catch is installed.

Magazine catch, spring, and button

Magazine catch, spring, and button

The catch installed in the lower.

The catch installed in the lower.

the spring is underneath

Magazine catch button in lower

After the magazine catch is installed, the official guide says to do the bolt stop, but I did this a little out of order.  It doesn’t really matter.  The fire control group is next.

The Trigger, trigger spring, sear, and trigger pin.

The Trigger, trigger spring, sear, and trigger pin.

The sear acts as a disconnect between the trigger and hammer

Trigger, sear, and pin as they appear in the lower.

STOP! hammertime!

hammer, hammer spring and retaining pin

When the rifle is fired, the bolt can reset the hammer regardless of the trigger position.  This ensures only one shot is fired for each pull of the trigger.

Hammer , trigger, and sear as they fit together in the lower.

hand drilling is safer for these small parts

fire control installed, the cocked hammer is visible as I open up the hole for the bolt catch pin.

i do not remember the official name of the little nub, and too focused on finishing this to go look it up.

bolt catch, spring, and nub

Any sideways torque might break the fragile printed pin-holder.

inserting the bolt catch pin carefully with pliers.

The bolt catch holds the bolt back when the magazine is empty, or when it is held down while priming.

with the bolt catch in place, I squeeze the pin through the other hole.

Next up is the front pivot pin which holds the Lower Receiver to the upper.

I drilling very slowly and backed it about every 1/8" or so, while pressing together with my fingers to prevent layer separation.

Carefully drilling the pivot pin holes. They were mostly the correct size as printed, but with the drill i can clean up any artifacts and make sure the pivot pin slides smoothly.

Pivot pin slides smoothely.

Pivot pin slides smoothly.

It looks in the picture like the layers cracked apart, but they are contiguous.

hand-turning this drill bit, I clean out the pivot pin retainer spring and pin hole.

Pivot pin, retaining pin and retaining spring.

Pivot pin, retaining pin and retaining spring.

 

This is the most annoying part of assembling an AR lower.  The retaining spring and pin must be inserted into the hole and held there while the pivot pin is inserted into the pivot hole.  It takes about 3.5 hands to do properly.

Pivot pin installed.

Can you spot the difference?

Buffer tower part of my two-part lower. An older version and the latest print.

 

not part of a standard lower receiver.

4mm bolt and nut which I use to attach the two parts of my lower together.

 

nut held captive in the front part of the lower.

nut held captive in the front part of the lower.

 

Two parts bolted together.

Two parts bolted together.

Now that the rear part of the lower is assembled, I can install the safety in the fire control group.

you can dance if you want to

Safety parts

The safety goes into the hole behind the trigger, just in front of my two-part attachment bolt.

takedown pin, retaining pin and spring.

takedown pin, retaining pin and spring.

Cleaning out the takedown retaining spring and pin hole

Cleaning out the takedown retaining spring and pin hole

Buffer and retainer visible in the lower receiver

Buffer and retainer visible in the lower receiver

Safety and takedown pin installed, along with the buffer assembly

Safety and takedown pin installed, along with the buffer assembly

Some would consider the lower to be complete at this point(minus the handgrip).  You may notice a captive nut in the handgrip area.  This nut I added to my printed lower after stripping the plastic threads of my previous versions of printed lower receiver.

Now I will add my combined grip and stock to the lower and have it fully completed.

My combined Grip and Stock attach to the printed lower.

My combined Grip and Stock attach to the printed lower.

Parts needed to finish attaching the grip and stock to the assembled lower.

Parts needed to finish attaching the grip and stock to the assembled lower.

 

A sling on this printed-lower-rifle would be unwise because holding it by a sling attached at the back and front would put force on the weakest area of the rifle, where i split my lower in two pieces for printing.

sling mount attached. I don’t need it to attach a sling, but it does have threads for the nut that holds the butt in place on the stock.

Butt attached to stock

Butt attached to stock

 

did anyone notice I changed my shirt?

Attaching an upper receiver first with the front pivot pin.

 

Closer on the pivot pin.  It slides so smooth like butter.

Closer on the pivot pin. It slides so smooth like butter.

 

Attaching next with the rear takedown pin.  filing down the mating surfaces of my two-part lower made this pin also slide in very smoothly.  Previous printed lowers had been very difficult to attach here.

Attaching next with the rear takedown pin. filing down the mating surfaces of my two-part lower made this pin also slide in very smoothly. Previous printed lowers had been very difficult to attach here.

 

The Complete Rifle. This is a .22LR upper, but when I next hit the range I'll be testing my .45 upper

The Complete Rifle. This is a .22LR upper.

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12 Responses to AR15 Two Part Lower Assembly

  1. Ben says:

    That redesigned handgrip/buffer tube area looks a whole lot better than the previous version. Just a couple quick questions:
    Which bits did you use to drill out the various holes?
    Are you going to post the files for your new designs?

    Thanks for this post though! It looks like a pretty good project to undertake. I’m going to try after I move (Spring)

    • turomar says:

      3/16 for the bolt catch roll pin and retainer springs
      9/64 for the safety detent
      17/64 for the pivot and takedown pins
      files posted.

  2. eddie says:

    awesome job. thank you!

  3. Mike says:

    It looks like you made this to take a rifle buffer tube, is that right? A1 or A2?
    Thanks,
    Mike

  4. Josh says:

    I know this is a lazy question, but i gotta ask. BOM available for the build?

  5. Darren says:

    Great job and thanks for sharing your hard work. Ive printed all the parts and I’m now looking for the hardware to attach/combine the stock?

    • turomar says:

      I used 5/16th threaded rod through the lower part of the stock. I chucked the rod in my cordless drill and ran it from the back until it poked enough out the end to get a nut on. Then i marked it, backed it out, and cut it. Then screwed it back in and onto the nut, using two nuts jammed on the back to apply torque. The buffer tube and butt plate hold the rest of it together.
      There are probably better ways to do it, but thats what I had on hand, so I designed around it.

  6. Darren says:

    Tested today, fired 50 rds of brass without a single jam. Lower printed on Makerbot in PLA. I didn’t use your combined stock, I had trouble keeping the hardware from pulling the buffer tube down when tight. Standard carbine stock and tube worked fine. Thanks again

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