Posted by: reformedmusings | August 23, 2009

6.8mm SPC AR-15 Build

I don’t normally build my own rifles, and I’m not a fan of the M-4/M-16/AR-15 design. Neither am I a fan of the 5.56mm/.223 caliber for other than varmints or plinking. I am a big fan of 7.62mm x 51 (NATO)/.308 for combat and hunting, especially in the M-14/M1A platform. So you’d think that an AR-15 would never find its way into my collection.

However, I’ve been toying with a variety of 6.8mm carbines for a couple of years. I’ve fired a number of commercial ones at various shoots. Dr. Gary Roberts, whom I’ve met and with whom I’ve corresponded, strongly favors an intermediate cartridge. You can read one of his ballistic study presentations here. He argues that the 6.8mm is an excellent compromise for combat (see slide 13 of his briefing). This matches historical experience (last slide at link) that the ideal combat caliber is between 6.5 and 7mm.

Late last year, a friend offered to sell me a 6.8mm upper receiver assembly for an obscenely low price. He had two and didn’t need both. I also learned that the lower receiver is the only serialized part on an AR-15, so I could swap upper receivers to shoot different calibers, including the .50 Beowolf. (.50 Beowolf is an interesting load that makes big holes but whose recoil in an AR platform is much like a 12 gage shotgun – very nice.) After much consideration, I decided to build an AR-15 around the 6.8 upper.

The “Obama Effect” made AR lowers almost impossible to find since just before the disastrous election last year. It took from last winter until last month to get a Territorial Gunsmiths, Ltd., high-quality lower receiver. I used a DPMS lower receiver parts kit as well as their 6-position adjustable stock. The upper receiver is a Lewis Machine Tool (LMT), Inc., Close Quarter Battle (CQB) carbine Monolithic Rail Platform and a 6.8 mm, 1 turn in 10″ twist, free-floating, chrome-lined barrel. Mine came with the bolt, bolt carrier, and charging handle installed. It also came with a pre-calibrated torque wrench and torx bit for changing the barrel.

The only “special” tool I used in the assembly was a roll pin punch set from Brownells. I tried driving the roll pins with a generic standard punch, but that didn’t work out well. The possibility of scratching the receiver was too high for my tastes. I did not use roll pin starter punches or alignment tools, nor did I need them. I simply held the bottom of the punch together with the pin in my fingers and manually kept them aligned with the hole in the receiver while driving it with a hammer. That worked without fail.

I assembled the lower receiver in an evening using the instructions at ar15.com, which worked great. The DPMS lower kit parts fit the Territorial Gunsmiths’ lower receiver perfectly. The roll pins went in much easier when lubed, and with one end squeezed down/narrowed slightly with pliers. I highly recommend using nylon strapping tape as suggested for the bolt catch assembly. I used 3 layers of simple packing tape and still put a small scratch in the receiver.

I did not have a pivot pin installation tool as shown in their illustration:

Illustration from AR15.com

Illustration from AR15.com

I simply used a very fine-tipped flathead screwdriver to hold the detent and spring down, installed the pivot pin with the groove away from the pin, then rotated the pivot pin until the groove caught the detent. Worked perfectly and didn’t scratch anything.

One shortcoming that I found in the instructions at ar15.com is that they don’t have an illustration for installing an adjustable stock. There are many varieties, but a typical example would have been helpful. I figured it out, but screwed the stock one turn too many into the receiver, which prevented the upper from closing properly (takedown pin would not insert). Backing the stock off one turn worked perfectly.

I used a Thunderbear 1-point tactical sling. It was a no-brainer to install and works great.

I lubed all the internal parts, and especially the takedown and pivot pins, with Gunzilla for function testing when assembled. I like Gunzilla because it’s non-hazardous, doesn’t smell bad, and cleans, lubricates, and protects very well. In fact, Gunzilla is the best cleaner I’ve ever used – even better than the old standby Hoppes #9. AR-15s like to be well-lubed. After the Gunzilla dries out, I may switch over to Otis Special Forces Dry Lube for day-to-day shooting while still using Gunzilla for cleaning. Otis SFDL is what I use on my other stuff and it works great. Plus, it doesn’t attract dust, dirt, or sand. Since the suspension liquid quickly evaporates leaving the dry lube behind, the firearm doesn’t leak oil onto clothing, etc.

Net result of the build:

AR-15_left_side

AR-15_right_side

The AR-15 turned out great and function checks perfectly. I bought C Products magazines for it. I plan to use Silver State Armory OTM 115 gr. ammunition when it comes back into stock.

I am ordering an Aimpoint Micro T-1 optic with a LaRue quick-disconnect mount for carbine:

micro_plus_mount

I have shot this optic on a number of weapons over last the few years. I was nailing targets at 400 meters using it on an H&K G36 with a 100 meter zero in Sweden with no problems. Aimpoint had it on an AK-47 at NDIA range day in Vegas this year, and it makes the AK a useful weapon. At just 3.7 ounces with its  stock mount, it keeps lightweight carbines lightweight, yet remains Aimpoint tough. The batteries last 5 years with the optic left on a useful daylight setting continuously. Sweet.

That said, I’d also like to have a Trijicon ACOG, but can’t afford one right now. These are outstanding optics which the USMC uses to great advantage. Most come calibrated for 5.56mm, but I’m sure that 6.8mm ones will follow in time.

Range report to follow…

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Responses

  1. […] SPC AR-15 with Aimpoint Micro T-1 I discussed my AR-15 build in this post. Well, I ordered and received my Aimpoint Micro T-1 Red Dot optic. Because the Micro is so small, […]

  2. […] was able to get to the range today to fire the new AR-15 I built. I discussed it construction in this post, and the optic in this […]


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