Building a 6.8 SPC II

6.8 SPC II Upper Built in this Post on an S&W M&P15 Sport Lower

Before I get started discussing this cartridge’s pros and cons, I must admit in the interest of transparency…that I am a 6.8 SPC II lover.  That love is based upon fact and familiarity, but I have a special place in my chubby little heart for this cartridge…It is the little cartridge that could…and it is my personal pet cartridge.  Now that I’ve completely ruined all sense of objectivity, and you still trust this post then lets continue.


Left: .308 Win          Center: 6.8 SPC II           Right: .223 Rem


       Left: .308 Win         Center: 6.8 SPC II (My Handload)           Right: .223 Rem

The quick and dirty history of the 6.8 SPC II:

The 6.8 mm Remington Special Purpose Cartridge (aka 6.8 SPC, & 6.8X43mm) was developed by Remington Arms in conjunction with SOCOM (United States Special Operations Command).  The purpose was to develop a round that out preformed the 5.56 round for both Close Quarters Combat (CQB) all the way up to combat distances of 500 meters focusing in the areas of barrier penetration, effectiveness on soft targets, and delivery of superior energy on target.

The developers tested the 6.5 mm projectile, which was found to have superior accuracy, as well as the 7mm projectile which was found to inflict the most damage.  The compromise was the 0.277″ or 6.8 mm projectile (for you fans of the metric system) which is the same diameter as the .270 Win.  The 6.8 mm projectile didn’t lose much in the way of accuracy or calamitous destruction from the respective test subjects. When it came to brass, they necked down the .30 Remington case because it fit in the AR-15 platform.  Thus, the 6.8 SPC was born.  The military apparently did not use the 6.8 SPC as a replacement cartridge for the 5.56 (probably politics or perhaps the cost of making the switch killed the idea).  However, for the purposes of this article (and my hunting season), I don’t give a poop if the military adopted the round or not, because by developing a harder hitting cartridge for combat, they inadvertently developed a great hunting cartridge for yours truly.  The original chambering of the 6.8 SPC was later modified and is referred to as 6.8 SPC II, which is the variation with which we hunt and shoot today. It’s a long story, but basically Remington screwed up on the specs submitted for original chambering that were adopted by SAAMI as the 6.8 SPC .  The 6.8 SPC II (which to my knowledge has not been adopted by SAAMI) is the corrected version of the chambering as well as the better preforming of the two.  It is this chambering that is readily available for purchase from the vast majority of barrel manufactures.  I wonder what happened to that engineer who submitted the wrong reamer dimensions on the drawing???.  As we know, the cartridge developed an avid following among hunters due to the multitude of positive attributes that the cartridge affords a hunter of beasts.

So, what’s so good about it?

I’m so glad you asked.

(1) A wide range of ammunition choices are available factory loaded from several manufactures, including but not limited to (in no particular order) Remington, Hornady, Silver State Armory (SSA),  Sellier & Bellot, American Eagle, Nosler Defense, PPU, Federal Fusion, Federal and there are probably more. As with any cartridge, the round really reaches its full potential if you handload ammunition, but fear not if you don’t because the factory ammo available has proven to preform quite well.

(2) Another plus is that the only thing needed to convert a standard AR-15 to a 6.8 SPC II is a barrel (and muzzle device of 0.277″ or larger), a 6.8 bolt, and magazines.  If you are building one from scratch, then all of the standard Milspec parts for your standard AR-15 build will work with the exception of the the barrel (& muzzle device), bolt, and mag, so there are still a ton of versatile options out there for your build.

(3) The round is better suited to hunting than the standard .223/5.56.  As we all know the .223/5.56 will kill things, but I like to leave a blood trail that Ray Charles could follow, and for that, exit holes are quite nice.  The 6.8 can deliver anywhere from 44% to 50% more energy to a target than a comparable 5.56 round.

(4) It preforms well out of shorter barrels, so if you are wanting to run an SBR (Short Barreled Rifle) or pistol, then you’re in luck. I personally use a 16″ barrel, and think it’s a nice compromise between velocity, weight, and maneuverability.  Admittedly, the .300 AAC (aka .300 blackout) probably has the 6.8 beat in the efficiency of powder burn in an SBR as well as in the sound suppression department.   However,  if you do not intend on using an SBR or suppression device (both of which require a separate Tax Stamp from the BATFE, and are considered National Firearms Act [NFA] firearms), then it shouldn’t matter to you.  If I were considering hunting with an SBR/Suppressor, I might still consider using the 6.8 over the .300 simply due to the fact that the 6.8 is much flatter shooting (if not using subsonic rounds), and makes it easier to shoot at some distance without having to compensate as much for bullet drop, etc.

(5) Accuracy out to 500 yards is great if you do your part, yet the cartridge is still able to deliver respectable energy to the target.  Therefore, within realistic hunting yardages, this cartridge is plenty accurate, and can deliver more energy than its .223/5.56 counterpart.

(6) It is very low recoil, which makes it a pleasure to shoot for me or my kids.  Kids can handle this rifle much easier than a .270, .308, etc., and yet still have a viable hunting cartridge. My 12 year old daughter, who is built like a sugar plumb fairy, can handle this rifle quite well.  I don’t have another rifle caliber with this low of recoil that I would consider having her use in a hunting situation, and still be as effective on a game animal.

What’s not so good about it?

(1) As mentioned above, if you want to use an SBR or run a suppressor for the 6.8, there are better options out there (in my opinion), such as the .300 AAC.  The 6.8 will still work quite well, but just a little better with the .300 AAC, so this “negative” is not all that negative.  I’m just saying if you are an SBR/Suppressor kinda hunter, then the 6.8 isn’t bad, but the .300 is probably a better choice.

(2) The factory ammunition can sometimes be a bit expensive when compared to .223/5.56, or even the 7.62X39mm (also know as the 7.62 Soviet, which is the primary cartridge for the AK-47, and variants).  Although the rounds for the 6.8 are much more readily available, and usually at better prices than the 6.5 Grendal or the .458 SOCOM. Because of its popularity, more & more manufactures are producing ammunition for this great little round, so the prices have continued to drop.

(3) The accuracy of the 6.5 mm rounds that will fit in the AR-15 platform such as the 6.5 Grendal are inherently more accurate especially at longer ranges than the 6.8 SPC (even though some will beg to differ).  This is marginal at realistic hunting ranges, so this “negative” aspect of the 6.8 is somewhat diluted.

So comparatively I have come to the conclusion that the  6.8 SPC II is the best all around choice for a hunting round that can be fired from the AR-15 platform if the hunter does not plan to utilize an SBR or run a can.  As things change, which is inevitable when it comes to firearms, a new contender will knock out the champ, but currently the 6.8 SPC II remains the king of the ring.


Building a quality 6.8 SPC II for hunting is a simple process.  All of the parts are the same as the standard AR-15 with the exception of the barrel (along with .277″ or larger muzzle device), bolt, and magazines.  So those are the only parts needed if you are converting your standard AR-15 upper from a .223/5.56 to a 6.8.

If you are building a stand alone complete upper, then the parts needed are as follows (from muzzle to rear):

  • 0.277″ or larger muzzle device (Make sure that your muzzle device is the same thread count as your barrel as well as the correct inside diameter to accommodate the caliber you are assembling.  We used a .308″ diameter Birdcage for this build)
  • A crush washer.
  • A 6.8 SPC II barrel (We used a 16″ stainless steel 1:11″ twist for this build)
  • Gas block (make sure that the diameter of the gas block matches your barrel.  This build uses a .750″)
  • Roll pin for gas block
  • Gas tube (make sure that your gas tube matches the correct length gas system for your barrel.  This build is a mid-length gas system [which is 9″ from the gas port].  I do not recommend any shorter than this for reliability purposes if you can help it.  Longer is okay.)
  • Free-float handguard (make sure that your handguard length will cover your gas block because it looks kinda funny.  You don’t want your friends to constantly be telling you “Dude, your gas block is hanging out.”  I used a 15″ handguard with a monolithic picatinny rail using the KeyMod system).  Along with the purchase of nearly every handguard comes the barrel nut.  Most manufactures have a proprietary barrel nut, so research is necessary in some cases to utilize a system that is easy to assemble.  The model used for this build is inexpensive, and super easy to assemble without having to align the barrel nut with the gas tube, and you don’t even need to use an Armorer’s wrench, so this handguard primarily what I recommend to people.
  • Upper receiver (the one purchased by the friend that I assembled this rifle for purchased this upper receiver with the ejection port cover, and forward assist pre-assembled) I have assembled expensive, and inexpensive upper receivers.  I have yet to find any difference in reliability or accuracy.  Maybe in a competition rifle, but this is a build for hunting/field purposes, so get a forged stripped upper receiver for a good price, and spend some extra money on the barrel where it counts.  So far this philosophy has been good enough for the girls we go out with producing great accuracy and reliability.
  • Ejection port cover, ejection port spring, ejection port pin, ejection port pin clip (this particular upper came with these parts pre-assembled, so assembly will not be shown below, but there are plenty of tutorials on the good ol’ web.  This is usually sold as an ejection port cover assembly kit instead of having to buy each individual part)
  • Forward assist plunger, forward assist pawl, spring, and roll pin (these parts were also already pre-assembled on this upper receiver, so the assembly is not shown.  The forward assist assembly is also usually sold as a kit).
  • 6.8 SPC II bolt assembly, right handed (including bolt, extractor, extractor spring, extractor retainer pin, etc. I don’t think I’ve ever seen a 6.8 bolt sold where all of these parts were not already installed)
  • Bolt carrier, firing pin, firing pin retainer pin, bolt cam pin (if you are converting a .223/5.56, then all you will need to do is replace the existing bolt with the 6.8 bolt, and you will not need to purchase these parts.  Also note that many places will sell the complete BCG for the 6.8 already installed)
  • Charging Handle
  • Mil-Spec Viton O-Ring to go over the extractor spring (you can also use a 1/4 X 1/8 X 1/16 Labeled #60  or sometimes 006 O-Ring from the hardware store will also do the trick)  This part is not necessary, but is recommended to aid in the reliability of the firearm.

Here are the parts laid out:

All you need are a few simple hand tools, most of which you probably already have:

  • Crescent Wrench and/or Armorer’s Wrench (I would recommend buying or borrowing the Armorer’s Wrench because it is great to have, and can be purchased for pretty cheap.  This is one of two specialized tools that I recommend you purchasing or borrowing, especially if you will be doing a lot of work AR-15’s)
  • Pliers (these are not necessary, but they do help when installing the roll pin into the gas block)
  • Allen Wrench Set (aka Hex Key, Allen Key.  I would have available both metric and standard because you never know what will be called for in an assembly)
  • Loctite or something similar (this is not a “must have” but I like to use it on gas block screws, and on handguard parts.  Some people use Loctite on the barrel when placing in the upper reciever.  I never have, but it is a common practice)
  • Punch Set (the blue set in the picture is a cheap steal punch set from Harbor Freight or something.  The set in the red case is a roll pin punch set [meaning it has a small stop that sticks out of the punch that fits into the end of a roll pin making it easier to drive in] purchased online.  Either set is pretty cheap, and can be used for a multitude of gunsmithing and other uses)
  • Hammer (they have a plethora of “gunsmithing” hammers out there, but I have used this claw hammer for a ton of work on guns.  Just don’t swing the thing like a drunk Serena Williams, and you’ll be fine.  Seriously, if she got angry with a claw hammer in her hands, she couldn’t be stopped.)
  • Moly Fortified Multi-Purpose Grease (also not a “must have”, but it is to keep the barrel threads from seizing)
  • Upper Receiver Vice Block (this is a must purchase or borrow item.  You will use this to hold the upper receiver while you assemble everything [except install the ejection port, and forward assist].  I purchased the cheapest one I could find online, and have this on many builds with no problem.  This one came as a kit that includes the block for assembling a lower receiver as well)
  • Bench Vice (I don’t see how you would pull this off without a bench vice.  I use the poop out of my bench vice for all sorts of guy jobs, so this is not just for this build…Go to a cheap tool store, and buy a bench vice…It will put hair on your chest!)
  • Latex or Silicon Gloves (again, not a must, but I use them to keep things off of my hands, and to keep from depositing sweat or oil from my skin on gun parts.)
Tools for Assembling AR-15 Upper

Tools for Assembling AR-15 Upper

Cheap Bench Vice that I’ve welded back together.

If you are converting a complete bolt carrier group (BCG) from an existing AR-15 chambered in .223/5.56, then you must swap out the bolt for the 6.8 mm bolt. So you will need to remove the Firing Pin Retaining Pin with a punch or your fingers.


Remove the Firing Pin Retaining Pin with a punch or your fingers.

At this point the Firing Pin should slide out of the carrier.

Firing Pin Removal

Without the Retaining Pin, the Firing Pin slides out.

Then you can remove the Bolt Cam Pin.

Removing the Bolt Cam Pin

Pull out the Bolt Cam Pin

The bolt can now be removed by sliding it out of the carrier.  [Note that the following step is not necessary, but can aid in reliability of your AR] Before you install the new bolt for your 6.8 SPC II, I would recommend first punching out the pin holding the extractor on your 6.8 bolt.

Removing Extractor from Bolt

Punch out the pin holding the Extractor in place

With the Extractor removed:

Dissassembled 6.8 bolt

Extractor removed from the new 6.8 Bolt

Once the extractor is removed you simply slide the O-ring over the extractor spring.

O-Ring on Extractor Spring

Installing O-Ring over Extractor Spring

O-Ring installed on Extractor Spring

Then reinstall the extractor, and retaining pin in the bolt.  To reassemble the BCG, do the dyslexic version of the BCG disassembly shown above, but this time using the 6.8 bolt.

Then get the Gas Block Roll Pin, and hold it lightly with a pair of pliers (needle nosed works well).

Installing Roll Pin in Gas Block

Roll Pin held with pliers

Slide the Gas Tube into the Gas Block, making sure that the gas port is facing the correct direction within the Gas Block.  Then align the hole where the Roll Pin is to be installed, and hold the pin in place with the pliers.

Installing Roll Pin in Gas Block

Holding Roll Pin in place for installation

Once you get the Roll Pin started in the Gas Block, then you can switch over to an appropriate sized punch.  Note that a roll pin punch may be easier to use here, but if you don’t have a roll pin punch, then a regular punch that you get from the hardware store works just fine.  Just be careful not to whack the snot out of the gas block and mark it all up.  Go slow, and in the immortal words of Happy Gilmore, just “tap, tap, tap it in.” Also be mindful not to let the Gas Tube get out of alignment while tapping the roll pin through (because you will just be beating on the roll pin like a red headed step child, and it won’t go anywhere. Plus, you can damage the gas tube if you are banging on the roll pin with the gas tube misaligned).

Installing Roll Pin in Gas Block

Tapping Roll Pin through Gas Block

Installing Roll Pin in Gas Block

Another angle with my chubby digits moved out of the way

Gas Tube installed in Gas Block

Gas Tube held in the Gas Block by Roll Pin

Next [not shown in this tutorial because it came pre-assembled] install the ejection port cover, and the forward assist.  Then place the upper receiver in whatever vice block you are using to secure the upper during assembly, and place it in your bench vice.

upper reciever in vice block

Stripped Upper Receiver in vice block

Next, slide the barrel into the upper receiver, positioning the barrel indexing stob in the recess cut into the receiver threads.

slide barrel in upper receiver

Install barrel into upper receiver

install barrel into upper receiver

Slide the indexing stob on the barrel into the recess in the upper receiver

Now we’re gettin’ somewhere, cause we gots us a barrel on this here humb-dinger!  Next apply a small amount of moly grease to the barrel threads on the upper receiver.  Don’t go crazy with this stuff, just enough to coat the threads and keep the barrel nut from seizing on the receiver threads. This is not an absolute must, but it sure does make a huge difference if you ever have to remove the barrel nut (to fix something, change handguard, or barrel, etc.).  Over years of field use, these threads can form rust, and without some kind of anti-seize, it makes the removal of the barrel nut (and thus, the barrel), very difficult.  The jar of moly fortified multi-purpose grease shown below was a few bucks at an auto parts store, and has enough moly grease to do 10,000 rifles, so it was worth it to me.  If you are only doing one rifle, then just get a small tube of it.  This stuff has worked for many a rifle that I have built, so I don’t plan on changing any time soon.  I’ve been told not to use any type of anti-seize/grease that contains graphite…something about galvanization….I’m not exactly sure what that means as it relates to my rifle, so just don’t use anything that contains graphite & your rifle won’t be visited by the galvanization gremlin.

Moly Grease

Moly Grease

Anti-seize greas on barrel threads

Apply moly grease to upper receiver threads

Install the Barrel Nut, and hand tighten.  The Barrel Nut is usually provided along with your handguard.  Most of the time, the company selling the handguard uses their own proprietary barrel nut, so these will look slightly different depending on the manufacturer.  I like this one because it is easy to install with a regular open ended or crescent wrench, and does not need to be aligned with the gas tube hole on the upper receiver.

Installing Barrel Nut

Install and hand tighten the Barrel Nut

Next tighten down the barrel nut using either an Armorer’s Wrench (some barrel nuts can only be installed with the use of a proper Armorer’s Wrench) or a crescent/open ended wrench of the appropriate size.  Every handguard has a recommended amount of foot pounds to tighten the barrel nut (some men’s barrel nuts can handle more foot pounds than others).  If memory serves, the recommended number of foot pounds is between 30 minimum and 80 maximum foot pounds.  However, I don’t have a torque wrench, so I Kentucky windage it.  I tighten it to somewhere between pretty tight to holy crap that’s tight.  In all seriousness, you don’t want this to be loose (this is what holds your barrel in your receiver, dude), so use common sense and don’t Betty White tighten the thing.  Keep in mind that you’re screwing a steal nut onto aluminum  threads, so don’t go Chuck Norris on the thing either, but it should be pretty danged tight. In conclusion, tighter than Betty White, and looser than Chuck Norris (who has no need for an AR-15.  He only invented the AR-15 to try to give regular mortals a fair fight…it is the only time that Chuck Norris failed).

Wrench on Barrel Nut

Tightening Barrel Nut

Next, slide the Gas Tube & Gas Block over the barrel.

Slide on the Gas Tube & Gas Block

Make sure to line up the gas port on the barrel with the corresponding gas port on the Gas Block.

Align Gas Port with Gas Block

Line up the barrel’s gas port with the gas port on the Gas Block

Then, slide the Gas Tube through the gas tube hole in the upper receiver (keeping the barrel’s gas port & Gas Block aligned all the while).

Gas tube in Reciever

Slide Gas Tube in Gas Tube Hole in Receiver

Then, tighten down the Gas Block using the Allen screws at the bottom of the Gas Block.

Tighten Allen screws, securing the Gas Block to the Barrel

Slide on the Crush Washer.

Crush Washer

Install Crush Washer

Hand tighten the muzzle device.

Screw on Muzzle Device

Put a piece of paper over the muzzle device so you don’t mar the metal or finish, and tighten with an Armorer’s Wrench or crescent wrench until the ports in the muzzle device are aligned so that the gas escaping from the barrel will push the barrel downward (if not properly installed, the escaping gas will throw the barrel in all kinds of crazy directions, thus making a follow up shot more difficult), so keep in mind that the escaping gas from the fired cartridge will move the barrel in the opposing direction of the ports on the muzzle device.

Paper used to keep from marring the finish

Orient the ports in the muzzle devise upward for controllable recoil

Slide on the handguard, making sure to align the picatinny rail [if applicable] on your handguard with the rail on the receiver.

Slide handguard over Barrel Nut

Tighten down allen screws with a little Loctite or the like

Put in your Charging Handle & BCG in the upper, and you are ready to slap that sinister hombre on your lower, and go hunting (after installing sights of some kind, unless you are Luke Skywalker).

Ready to Rock.  Man, I look hot in latex!

And yes, my work bench is cluttered.  In the words of a pretty smart dude, “[i]f a cluttered desk is a sign of a cluttered mind, what then, is an empty desk a sign?”—- Albert Einstein.  Thanks for the props, Albert.

The process of building a fantastic hunting upper that will give you years of hunting enjoyment, as well as the feeling of personal pride that comes with taking a game animal with a rifle that you assembled yourself from parts is well worth the small amount of effort involved.  It literally took me more time to put this post together while explaining the process than it did to build one of these.


Top: 6.8 SPC II built in this post.           Bottom: My friend’s .223 upper he originally had on this lower

Completed uppers are readily available for purchase, but you will (in most cases) spend a little more for the same upper that you can build yourself, and there will inevitably be some part or component on the complete upper that you will want to change, or that you dislike.  I have guys bring me rifles all the time who bought a rifle, then spend a small fortune tearing parts off and replacing them with what they really wanted in the first place. They end up spending more money than building it the way they wanted from the get-go.  By building your own, you can purchase every part exactly the way you want it.  You can also devote a larger amount of funds to the parts that are important to you, and lesser amounts to other parts.

Categories: hunting, reloading, Shooting

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1 reply


  1. **Video** A Girl’s First Whitetail – Taken with a 6.8 SPC II – Greatoutdoordinary

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