AR 15 6.8 SPC built and review

Hi all,
I wanted to share with you my experience building my AR-15 6.8 SPC. This is work in progress so I ll be posting as I complete the rifle. I will be reviewing the components and sharing my views and thoughts with you and would definitely invite your feedback and suggestions on components, tips and tricks.

Introduction/requirements:

For building a rifle I had the following requirements: I wanted a semi auto light deer/coyote hunting rifle and a 200/300 yard target rifle. Given my military experience in Europe I was initially contemplating the NATO 7.62x51 cardridge and I was also looking for the AR platform for its flexibility, availability and diversity of supported calibers. It also had to be fairly light so that I can easily pull it up a tree stand. I wanted to build a custom rifle from scratch so all the ready-made platforms (like the Kel-Tec, HKs, etc.) were out of the question so naturally my choice was between the AR-10 and AR-15 platforms. Also I want to support the US AR industry as I believe between them and the consumer’s AR craze they are doing our country a great favor. During a national emergency our country will be well stocked with parts and is virtually immune to interruptions of supply by any single manufacturer. Plus all the private consumer driven innovation and improvements to the platform eventually add to the improvement of the tools that our soldiers take to war. What other country has that?

Since last spring I was designing two rifles shifting back and forth between calibers, identifying components and pricing specs. This was whenever I had free time between work, babysitting and scouting my favorite public land hunting grounds in the area. So I was taking my time to make a decision, and at the same time I was also saving money for the future rifle.

My decision was made for me

One day last September I was chatting with my neighbor about his new guns and he mentioned that he was glad he got his AR before Oct. 1st and I go: “what happens Oct. 1st?” What was I thinking? How did I miss all the fuss? I was caught snoozing! People like me let this happen to our state. My decision was made for me. The next day I drove over to Engage Armament off of Gude Drive and got myself an AR-15 lower. I didn’t have the money for the whole rifle but I figured I could take my time buying components as I saved (I don’t like impulsive purchases and owing money on my cc).

First impressions:

I have never owned an AR-15 based rifle, I have never fired one. My experience when I served in Europe (where I originally come from) was with the German HK11 and G3A3/4 all in 7.62. My first impression when I got the parts for the lower half was that the thing is tiny. The mags are tiny, the rounds are tiny too. I spent my young years lugging around a 40 inch plus, 10 lb rifle (G3) and on occasion an 18 lb HK11. No matter what you do to an AR-15 whether it has an A2 stock or a 20 inch HBAR it is still a light/small rifle.

Lower receiver:

The $200 plus $10 transfer and some change on tax lower receiver was made by Engage Armament of Rockville, MD (probably by a contractor). I bought my pistol from them so they already had my info and safety class code. I live in Rockville and it’s nice that my address was already engraved on the lower .
Even though my preference is mil spec hard anodized forged 7075 Al, this lower is CNC milled from an aluminum billet (don’t know if its 7075 or something else). The lower is colored black, but I am not sure if it’s hard anodized as well. A couple of places I nicked it the color came off easily.
The benefit of a CNC milled billet is that you can get much better tolerances. I measured the locations of the pin holes and other reference points and they are mil-spec. The pins fit, and the milling job is really high quality. You cannot detect any end milling marks with the naked eye and the magazine well appears to have been broached. The beaver tail area where the thumb rests between the pistol grip and butt-stock/buffer tube has a smooth contour with no detectable threshold. The buffer tube threads were perfectly cut. The trigger guard was cut as part of the lower and is not removable. It’s pretty thick, but there is still more than enough space for my finger even with a glove. I actually like it this way as it provides better structural integrity between the mag-well and grip. Also this allows for a perfect fit of the plastic hand grip with no gap to catch your finger.
The mag well is flared for easy mag insertion and there is enough material on the mag well rim for structural integrity.
The markings on the lower were embossed CNC engraved. On the left side above the grip for the fire control group there is “safe” and “semi”. To the left of that there is Rockville, MD, USA (where I live) and over the mag-well “Engage Armament LLC” “My serial number”, and “E4 Multi” probably meaning multi-caliber as one can build the rifle using any number of calibers. On the right side of the lower above the hand grip for the fire control group again there is “safe”, “semi” and “engage” (get it? for Engage Armament…). Over the mag-well there is an outlined “e”, again for engage Armament.

Lower parts kit:

I ordered the DPMS lower parts kit without a trigger group ($30 from Brownells). Even though I didn’t need the hand grip nor the trigger guard, it is less than half price ordering the kit rather than individual components. The DPMS parts are ok, not the best quality, a bit gritty, but they fit perfectly on the lower and after some oiling the take down pings moved smoothly and the detent springs did their job as expected. The pins, fire group, mag release and bolt catch appear to be cast iron based alloy and coated grey.

Trigger group

I ordered the Rock River National Match 2-stage trigger and pins (~$120). It is also cast iron based, but it appears that their molds are of much better quality/detailed/high res than the DPMS (I should have gotten the Rock River LRPK instead…). The trigger surfaces at the engagement points between trigger and hammer have been hand adjusted and polished and the trigger is smooth and crisp right out of the box. The trigger has a light first stage (about 3 lb) and about 5 lb second stage with very little over travel. However, when you hold the trigger back upon firing to simulate bolt travel and recharging, your finger is kicked forward when re-cocking.

I also wanted to mention that this is the second trigger set I got. The first one I returned was the CMMG 2-stage trigger. They appeared very similar and I was drawn to it because of the brass bushings. The CMMG trigger concept and design had great potential especially with the brass bushings, but there was poor execution. The surface of the trigger was not smooth and it would need a lot of polishing to finish. The spring of the second stage was so hard that the pull felt like 8 lb (almost like shooting a revolver). The Rock River was the same price but better quality.

Grip

I don’t like the stock LRPK grip so I ordered the ergo pistol grip ($23 from Amazon). It has a soft rubbery feel over-molded on a hard plastic similar in texture and shape to the standard kit grip. I attached the grip with the standard flat-head stainless steel screw and some Loctite. The grip feels great in my hand and it fills my palm nicely as if it was made for my hand. The interface with the surface of the receiver is perfect and the fire control lever can move freely. The beaver tail lays perfectly on the receiver surface. There is rubber cover that stays in place like a wine bottle replacement cork that can easily store a round and a bore cleaning kit.

Buffer tube, buffer and spring

As you can see from the pics, I decided on the Magpul PRS stock as I like the potential for adjustment and that dictated my choice for a buffer tube AKA lower receiver extension tube. Based on the Magpul specs one needs the M16A1 length buffer tube. The first one I got was an Ultimate Arms A1 buffer tube, spring and rifle length buffer. Upon opening the bag and looking at the spring and buffer I know it was not going to work for me. First of all the buffer was made out of a thin aluminum shell filled with loose lead BBs rattling as you shake it. I questioned its reliability and design/quality. The second issue was that the spring touches the Aluminum buffer tube and as the buffer moves in and out there is a lot of friction and rattling around. It just screamed to me “bad design”. Without experience with the AR-15 A1 I could just imagine an annoying sprrooiing sound whenever I shot the rifle.

Excellent, another 6.8 guy. I'm looking forward to the rest of the story.

nice build.

Alright the wife and kids and are watching a movie and I discovered a bottle of barley wine I brewed 2 years ago, so I have about an hour and will try to finish the lower. It took me less than an hour to put the lower together but its taking much longer to write about it...

JP SCS Buffer/Spring system

After searching the web I did find others that had the same spring sound issue and they fixed it with grease. I was sure someone must have come up with something better, like a piston, or something like the Glock pistol recoil spring assembly. Then I found the JP Enterprises Silent Captured Spring. It’s a bit expensive ~$140, but if you think how much a Glock recoil spring costs its not too bad. Plus there are options for 5 different spring tensions so that you can tune your rifle for any type of load you have for another $40. This is the beauty of the AR platform, you can pretty much find whatever you imagine.

The buffer and spring system is a brilliant design! I its milled/lathed from stainless steel. The stainless steel spring is kept tensioned by a stainless steel retaining rod (captured). The bottom of the retainer rod (towards the shoulder) is capped by a fixed stainless steel cup that is screwed onto the rod by a flat recessed Allen type screw and marked “This end to the rear”. The top of the spring is capped by a free floating buffer assembly which is capped by an Allen type screw. This is the end in contact with the bolt carrier. If you push down on the buffer it slides down like a piston while the rod protrudes (just like the Glock recoil spring). As the bolt carrier moves into the buffer tube the retainer rod enters the hollow space of the carrier just behind the bolt and the firing pin.

The buffer basically consists of the spring cup and three stainless steel lathed buffer weights separated by rubber O-rings and kept in place by a ring tensioning spring. The rubber O-rings prevent the buffer weight rings from rattling. Because of the way the spring is captured and held in place there is no rattling, as it touches neither the spring tensioning rod nor the buffer tube. Furthermore friction is much lower between the rod and spring cup than aluminum on steel of the standard configuration. The JP spring system has the proper length for a carbine buffer tube length and it comes with a white plastic spacer to provide proper positioning for the A1 buffer tubes. The plastic spacer slides easily in and out of the buffer tube and it has a venting hole to accommodate the movement of the buffer.

Note that when you use this buffer/spring system the buffer retainer and spring are no longer necessary. In fact if you install them you risk getting the buffer stuck in the tube and having to remove the buffer tube to remove it.

Buffer/lower receiver extension tube

The buffer I ended up using is a DPMS A1 rifle tube ($16 from Brownells). It appears milled and turned and bored from a monocoque piece of aluminum and then painted black. The tube has an oval protrusion in the bottom so that it can be installed with the AR armorer’s tool. The threads are just long enough to properly cover the buffer retainer pin (if it was used) and cover the whole length of the receiver threads. I installed it with Loctite after cleaning with acetone and hand tighten it with the AR armorer’s tool without installing the receiver on a vice. The spacer and the JP SCS buffer/spring assembly slides right in once you lightly press the hammer down, and sits flush with the lip of the A2 buffer tube.

I appreciate the hard work and subscribed to this thread. Keep it up. Looking forward to the next movie that your wife and kids watch.

Nice build, another 6.8 guy.

I'm new to the AR in 6.8 also.
I think I'll start reloading soon.

6.8!! Woot! great choice!

Waiting to see what upper is coming.

I've been looking at a stag 7H found one for 575 shipped.

stu929 said:

Waiting to see what upper is coming.

I've been looking at a stag 7H found one for 575 shipped.

Click to expand...

If you look at the tags, you might see Bison Armory. My 6.8 has a Bison Armory upper. I think I'm picking up another after I finish with my 3-Gun equipment splurge.

Thanks every one for your comments and the vote of confidence on the 6.8 choice. I don't have the upper yet as I am still saving for it, but should be able to get it soon. I guess my tags were a spoiler, and I am thinking of using the Bison armory upper. I figured those guys specialize only on that caliber they can't help but becoming good at it after a while.

Jimbobborg you said you have one, how do you like it?

boss66tcode I am also realizing that I ll have to start reloading given the ammo and case prices... Never done it, so it will be a fun learning exercise!

Magpul PRS butt-stock

The last part to complete the lower half is the butt-stock. Why did I choose the PRS? For two reasons: its heavy enough to balance the 20 inch barrel I am planning to get and its adjustable enough to accommodate my shooting style. I have experience with the M4 carbine stock on my Glock CCU and it just feels too small and keeps my cheek too low (the Mechtech carbine conversion unit has the same geometry as the AR-15). Also I don’t feel comfortable shooting with a short stock even when standing and I keep mine always extended so I don’t care for a collapsible stock. Lastly, I feel the standard A1/A2 stocks are too sixties… (see Mel Gipson in “we were soldiers” )

The Magpul PRS stock is very compact, solid and heavy (almost 2 lbs, slightly more if you include the buffer). It is constructed from glass fiber reinforced nylon parts that bolt together to make the butt stock. This means that you can completely disassemble it and service. It is nicely textured and it includes two aluminum sling attachments near the receiver and near the shoulder and they are ambidextrous. You can adjust about ¾ inch for the chick height and an inch for the total length of pull. It goes from about -1/4 to +3/4 inch of the typical A2 length. The cheek rest has only one steel post that pushes it up and down. The post is positioned right below the buffer tube and attaches on the cheek rest which hugs the buffer tube on both sides resulting in a very sturdy construction and its solid even when fully extended. The movable butt stock is made from CNC aluminium and has a removable rubber butt pad that spares your shoulder from the recoil. The moving part of the butt stock is attached on two steel rods for added stability and strength. Both adjustment knobs are aluminium and they are the locking/click type so when you adjust to your preferred positions it stays there. The steel/aluminum adjusting mechanism, the aluminum butt plate and the really thick nylon is what contributes to its weight.

The butt shape allows one to hold it with their left hand when a bipod is used (for right hand shooters). It also has a hidden picatinny rail on the bottom for attaching a mini monopod for when shooting from the bench. It comes with a hollow mounting screw with thread lock. The stock is held securely from multiple points:

1. A nipple on the butt stock secures into the lower receiver indent
2. The hollow screw forces the stock against the receiver
3. Tightening the forward sling attachment bolts grips the buffer tube tightly

Effectively once installed properly it will not move and reinforces the strength of the butt stock/buffer tube system.

It has a high MSRP, but I was able to get it for $200 off of ebay.

Magazines

I was going to go for the cheapest ones I could find, as for some reason 6.8 SPC mags are quite expensive, but then I read a few reviews and when I discovered my favorite round (SSA 140 Grain Berger) they also recommend the Precision Reflex Inc. (PRI) mags. Unfortunately my indecision between 6.5 Grendel and 6.8 SPC pushed me beyond Oct. 1st so when I decided on the 6.8 SPC caliber I could only get the 10 round mags. I got two mags from Brownells for $80 (ouch!). As it is, they barely protrude below the mag well – they are tiny.

I keep telling myself that it doesn’t matter since the rounds are so expensive I won’t be shooting for quantity. In any case I can only load 8 rounds when I go hunting.
But why can’t I have 15 or 20 round mags which I think it’s the reasonable size mag one should carry on this platform. I am so glad I have 5 15-round mags for my Glock. The other day I participated at the Gilbert's range Hogan Alley event and I needed every bullet in the mag for the course without wasting ammo. I wonder for people that participate in shooting matches if there is any exception to the 10-round-mag rule, or if the event organizers give people from Maryland a handicap… Anyway, enough of my rambling, I wanted to put my complain out there…

Back to the PRI 6.8 SPC mags I got. They are very well and carefully made. PRI’s QA/QC works well. They are steel construction from 0.033 inch thick sheet. They have a waffle pattern as well as two vertical indentations for protection against bulging. Effectively they are made by stamping the two halves of the mag with a tool and welding them together at the locations where the primer and the tip of the bullet sit. The seams are very clean and smooth without any indentations giving a total length from primer to bullet tip 2.320 inch and 2.460 inch total length. Effectively the manufacturing technique allows them to achieve this large tolerance and that is why SSA recommends them since their rounds are 2.300 inch overall length. The mags have the PRI logo as well as 6.8 mm written on them. The plastic follower is olive green and has 6.8 inscribed near the bullet tip. The bottom of the magazine has the following stamped: "WWW.PRI-MOUNTS.COM, USA, NEW BREMEN, OH". When I am done with the rifle all the components will have the MADE IN USA logo on them (except for the scope...)


Regarding fit and function in the lower receiver: the mags insert effortlessly and catch on the mag catch.
When you press the mag release button the bolt catch spring shoots them out of the mag-well as expected. I haven’t loaded them with ammo yet to see how they do.