Sunday, January 7, 2018

Buffertube towers and the $2 receiver

Rants on the Flat AR Lower and Buffer Tube Interface Dilemma
Also – is a $2 lower receiver possible!? (The quick answer is yes, see halfway through post below)

Note: This article is more or less a continuation of my "Home-Depot" lower receiver series which can be chronicled here and here.  My interest in this project continuously waxes and wanes over time.  I wrote this back around June 2017 and added to it over a period of months so this article is kind of a disjointed mess, the links are a little old and the prices are not quite up-to-date.

An ideal starting point for a flat AR.
Concept by elmgrove1765
One of the problems faced with the non-milled AR lower (i.e. a lower made from metal flats, wood, or other laminates) is that no one specifically makes a threaded block to interface the buffertube to the receiver flats.  The Jack Squat Flat Lower requires a lathe to thread the buffer tower piece that comes with it, in addition to welding, so that alienates the typical garage builder who doesn't have access to a lathe or welder.  An AR15 buffer block (pictured at right) would be ideal for the home builder because you could just drill & tap the holes of the sideplate and screw it together.

Recently 80% lowers or even 100% lowers have come down as low as $25 in the past month.  So why would anyone need to cobble together a flat lower when you can get something solid for cheaper? Several reasons:
  • A cheap “throw-away” receiver can be smuggled inconspicuously in pieces and the upper sent through the mail (this would probably only make sense in the US/Canada since in most other countries the law considers that the barrel is the gun, not the receiver)
  • Rapid mass-production of lowers using cheap scrap or sheet metal in an underground, budget workshop
  • Milling out solid lower receivers on a drill press is vastly more time-consuming compared to drilling holes on precut pieces of thin metal sheets.
Stock adapters, left to right: Rifle Dynamics, KeepShooting,
Shockwave, and USMachineGun
 So here is quick rundown of threaded stock adapters for various other weapons where the user would want to use a collapsable M4 stock, but these also happen to work as buffertube towers (or buffer trunnion, if you want to call it that) for our flat lower.
  • Clamp-on, single point sling attachment adapter (requires two castle nuts).  Aluminum.  This is the cheapest solution at $3 for the adapter plus $1.80 for each castlenut, totalling $6.60 for the assembled unit.  This is used as an attachment for adding a sling to a buffertube.  It has a cup where it would slip over the castle nut, but as seen right, it can be modified and used to interface the flats to the buffertube.  The sling loops may need to be cut off, but I got away with leaving them intact.  I’ve actually rigged something even more ghetto (see further below) that doesn't require castlenuts or adapters, but this is the cheapest thing commercially available if you don't want to drill up a buffertube.
  • Another Clamp-on Sling Mount - $8.70, aluminum.  Comes with built-in sling mount that will need to be cut off.  Also requires a castlenut.  Seems better made than the one above, but I have not tested this out.
  • MAC-10 stock adapter of unknown origin - I like these - they’re steel, they’re thick and they’re $30.  I don’t know who makes them or how much KeepShooting has left in stock, but they look nice.  They have flat sides and with plenty of room to mount screws onto them.  KeepShooting has 15% off codes every couple of months during special holidays, so that's worth considering.
  • Shockwave Mighty Mount M11 adapter - $35, aluminum, and it has wide surface area for sideplate attachment options.  Might have to re-profile it to mate with the upper, or you could install it backwards.
  • AngryGunAirsoft SCAR adapter - $40, aluminum.  It’s for airsoft but it uses standard AR15 threads.  It’s a bit too long, length and height-wise, but it seems like it could be put to good use with the right modification.  A bit expensive for me to risk experimenting with dremelwork, but I like the wide surface area.  This could be the basis for some kind of takedown ar15 where the buffer assembly can slip on and off.
  • Midwest Industries Heavy Duty QD Sling Adapter - $50, aluminum.  Wide surface area and lots of metal.  Might need a buffer tube with lots of threaded space.  Have not bought or tested this.
  • RifleDynamics AK adapter - $65, aluminum.  No reason to buy this, but it’s there if you want it.  I don’t think there’s enough metal for my project’s intended use.  It's a really nicely made adapter for the AK.
  • USMachinegun M11 Adapter - $66, aluminum.  This is a bad price for what this is.  Even as an actual M11 stock adapter, it feels like something that was made in China and should be sold on ebay for 10 bucks.
  • photo by backbencher,
    Hera Arms CQR stock
  • Hera Arms CQR stock - $120, polymer.  I am definitely not spending 120 bucks on a throw-away receiver - but again the option is there if you’re into that, I guess.  This idea was from a fellow on the Ar15.com forums named backbencher who posted a picture of an AR15 FCG testing fixture connected to a buffertube via this stock, with an upper mounted to it.  He planned on putting a 5.7mm P90 upper on it, which would solve the problem trying to fit a STANAG magazine well.  This stock is interesting in that it could be useful for repairing AR lowers where the buffer tower has snapped off.
Finally, this is an even cheaper option than the $6.60 clamp-on sling adapter with castlenuts assembly that was first mentioned above.  Using the following method, the receiver doesn’t cost anything but the price of the metal you use.  I am not going to factor in the cost of a carbine buffertube since you would presumably be buying one anyways, however, it does require cosmetically permanent modification to the tube.  Carbine buffertube kits right now sell for less than $20 on ebay or Amazon.  Also check Chinese wholesale websites like DHgate where it’s currently 14 bucks for the kit - that is if you don't mind waiting a month for delivery.  Or you could just get the tube by itself which I've seen sell for 5 bucks at Copes Distributing once.

A six foot length of 1/8” x 1.25” aluminum costs about $6.00 from Tractor Supply, which you could make several lowers from.  Steel would be better, but I wanted to try aluminum first for ease of drilling and cutting.  You simply bolt on the long sideplates to the stock adjustment rail, and locate the holes for the front and rear takedown pins and FCG (I go in depth about aligning the tube with the upper and locating the holes on the sideplate in Home Depot lower part 2).

Step 1: Add the 1/8" x 1/2" aluminum bar to one side (I think it's half inch, might be 3/4").  Make sure it's long enough to accomodate the rear takedown hole.  It probably didn't need to extend all the way to the end like that but you probably will need to bend it later to keep it parallel with the upper.  You can use Cleco pins, small C-clamps, vise-grips, and locking pliers to hold everything in place until you screw it together.

Step 2: Use 0.990" to 0.999" dowel/rod to align the upper to the buffer tube.

Step 3: Close them together snug.

 Step 4: Check other side.  Make sure top edge of the metal bar is parallel with bottom edge of upper.  If not, you can bend the bar a little and screw it in place to the buffer tube.

Step 5: I used a short length of 1/4" tubing with a 1/8" hole to center the 1/8" drill bit.  From there you can use a 1/4" bit to widen the hole in the plate.

 Step 6:  I recommend using 1/16"  x 1.25" steel here to add rigidness and strength to the takedown holes OR 1/8" thick aluminum.  The photo shows 1/16" thick aluminum which was too thin and caused egging of the hammer holes (not shown).  It also serves as a place to drill the fire control group axis pins to.  Remember to keep it parallel to the upper.
 
  Step 7: There it is so far.  Repeat the process for the other side.  I think you can figure the rest from here.

I never got a chance to make a live-firing model, but I did make a proof-of-concept while I was on leave in May.  This was to see if I could create a stable platform to interface the buffertube to the upper.  To test alignment I installed the charging handle, bolt carrier group, spring and buffer.  With the rear takedown pin in place, the thing is solid.  You can pull the charging handle and feel the bolt-carrier ride smoothly in the buffer tube without any hiccups.

Another idea I had was to attach this to my partially complete Jack Squat Flat lower and just do away with the buffer piece that comes with it since I have no way of threading it.

Further reading:
The Home Depot Lower Part 1 - Video
The Home Depot Lower Part 2 - Instruction
Winter Soldier’s semi-auto, wood and metal lower - I drew some inspiration from here.
3d printed AR lower - 3d printable receiver in small sections, bolted together.  Perfect for small 3d printing machines, I would think.  If someone can produce the threaded buffer block and sell them to me that would be fantastic.

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