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An AR Purpose-Built for Hunting March 2021
Dan Martinez

I’ve taken after piggies a couple of times with my 7.62x39 Colt AR, so far unsuccessfully. Recently, I’ve been working on a new AR build. Not for plinking and play, not for competition, not for self-defense, not for varminting, not for SHTF, not for the zombie apocalypse; but specifically, for big game hunting.

It’s been a back-burner goal of mine to take game with an AR, something that I still have not yet done. The goal remains open today, after a deer season and a javelina season have passed since I started this project. The deer season passed without success because the rifle wasn’t quite ready. The unsuccessful javelina season was strictly due to the piggies not showing up when I needed them to.

OK, when I say that this is a big game hunting AR, I mean specifically those two critters, javelina and deer. I was not interested in an AR-10 platform rifle, which might also allow me to think about going after elk. An AR-15 is lighter and easier to carry afield than an AR-10.

So that decision right there limits the possible cartridge power level to those that remain under 2.26” in overall length. There have been a number of recent cartridge developments that have allowed the AR-15 to become a bona fide deer rifle out to at least the 200-yard mark. By that I mean punching a hole bigger than 5.56mm and carrying in the neighborhood of 1000 ft-lbs. of energy to that distance or farther. Some of those recent cartridge developments include the 6mm ARC, the .350 Legend, the .300 BLK, the 6.5mm Grendel, and the 6.8mm SPC.

It was the 6.8 SPC that caught my eye some time back. I actually bought a box of 115 gr. American Eagle 6.8 SPC before I had anything to shoot the cartridges with. When I did clock this load, I saw 2568 fps for muzzle velocity. Running the ballistics, I was surprised to see that this load carries at least 1000 ft-lbs. to 270 yards! Out of an AR-15!

I was strongly considering getting a 6.8 SPC rifle barrel for my Contender frame. But then I would also need to get a carbine buttstock, a carbine fore-end, a scope mounting base, rings, scope, the list goes on.

But then I saw Palmetto State Armory advertise an 18” upper in 6.8 SPC that looked to be exactly what I would want to start a 6.8mm AR hunting rifle build. I could put it on a lower that I already had, potentially saving some money, and end up with a repeating rifle that the 6.8 SPC was intended for, instead of just a single-shot.

Yeah right. You know that the saving money thing never works out. I wish I could say that this was as easy as tinker-toying the rifle together, sighting it in, and going hunting, but the truth was actually much more painful.

The Cartridge
 
.223 Rem/5.56mm, 6.8mm SPC, 7.62x39mm

Before getting further into my build experience, let me talk in general about the 6.8mm SPC. Unfortunately, the commercial rollout of the round was botched, and we have to live with the ramifications still today.

In 2003, members of the Army’s 5th Special Forces Group and the Army Marksmanship Unit collaborated to design a new cartridge which would be a significant improvement in lethality over the 5.56mm NATO round. This cartridge was intended for use in the Mk12 Special Purpose Rifle. The SPR was an “enhanced carbine,” chambered in 5.56 NATO, which filled the role of a Designated Marksman Rifle for the Special Forces. The military wanted a cartridge with more punch for this DMR, which was an M16/M4 based platform.

After extensive testing of various calibers between 5.56 and 7.62mm, the Army team chose caliber 6.8mm as the sweet spot. To American sportsmen, this bullet diameter is better known as .277”, used by the .270 Winchester since 1925. Being a smaller cartridge, the 6.8mm SPC would use lighter bullets than are typically used in the .270 Winchester, but significantly heavier than the 77 grain Mk262 5.56mm round being used in the Mk12 SPR.

Similarly, after considering various case sizes, they chose the 1906 vintage .30 Remington cartridge as the basis for the new Special Purpose Cartridge. The .30 Remington was originally designed as a rimless competitor to the Winchester .30-30. It was the happy medium in size between the 5.56mm NATO and the 6mm PPC case sizes.

The 6mm PPC was derived from the .220 Russian, which was derived from the 7.62x39. These cartridges all share the head size of .445”, as does the 6.5mm Grendel and the 6mm ARC (with small differences).

For the military, there was concern that this larger head diameter removes too much material from the locking lug area of the bolt of the M16/M4. The fear was that this may lead to reliability problems due to the lugs cracking. The head and body diameter of the .30 Remington is .422”, providing greater powder capacity than the 5.56mm with a head and body diameter of .378”, but better perceived reliability than the larger 7.62x39-derived cases.

The Army team then turned to Remington to further develop the cartridge for mass production. This is where the FUBAR occurred. When Remington finalized the spec for the chamber reamer, they ended up specifying the leade, or freebore, too short at .050 inches. As a result, Remington could not produce ammunition that achieved the velocity goal set by the Army without running into pressure limits. This failure effectively sealed the fate of the SPC with the military. They never adopted it.

The faulty reamer spec was submitted and approved by SAAMI before anyone realized the problem. So basically, the original SPC spec is now written in stone. Commercial ammunition must be made to fire in SAAMI chambers, and is therefore velocity-limited to stay within safe pressures.

Specialty manufacturers and the user community then picked up Remington’s dropped ball and ran with it. A “Spec II” reamer design was developed that increased freebore to .100”. Additionally, the twist rate was slowed from 1:10” to 1:11”, further reducing pressures. A faster twist is not needed for the shorter and lighter (than the .270 Win.) .277” caliber projectiles used in the SPC.

However, the 6.8 SPC II spec is not recognized by SAAMI. This means that handloaders can achieve the cartridge’s original design parameters of up to 200 fps greater velocity compared to commercial ammunition, but ammo companies cannot.

The downside is that unless a user has some way to measure pressure, it’s a bit of a guessing game once you go beyond published load data. The published data, just like everything provided by the industry, must adhere to the original SAAMI spec.

Most every rifle and upper produced today is made to the SPC II spec. There is no harm in gun manufacturers producing Spec II guns since the result will be lower pressure when using SAAMI compliant ammunition.

The Upper


These are the full specs for the upper. When I ordered it, the cost was $360, including bolt, bolt carrier, and charging handle. Seemed like a great deal. One thing that I liked was the medium-weight A2 barrel profile. Not too heavy, not too light. I wanted to stay mindful of weight on a rifle whose mission it was to be carried through rough terrain. I particularly liked the lightweight free-float handguard that did not needlessly continue the Picatinny slots over the full length of the tube.

First Shots with the AR-6.8
Whether I was starting with a 6.8 SPC Contender barrel, or with an AR-15 upper, at this point I needed more parts! I needed to get magazines, a scope mount, and a scope to get started shooting my 6.8 SPC upper. Though in a pinch you can use standard 5.56mm magazines, for optimum reliability, magazines specific for the SPC have a different cartridge follower and guide ribs.

Then I needed everything to handload for the 6.8; dies, cases, bullets. The primers many resources on the web recommended were the white box CCI #41 military style small rifle primers for 5.56mm.

The best deals for bullets were from Speer. For plinking and practice, the 90 grain Speer TNT hollow points were quite affordable. These are varmint bullets. For game hunting bullets, it was Speer Gold Dots in both 90 grain and 115 grain weights. These were also quite affordable, yet they are bonded soft points that many users reported great success with hunting hogs.

I assembled the upper onto an Aero Precision lower with an A2 buttstock that I had put together several years ago for a heavy barrel 18” 5.56mm upper that I had also purchased from Palmetto State. I had installed a very nice Elftmann trigger on this lower.

Since the deer season was coming up, using book data, I created a ladder of loads for the 90 grain TNTs and the 115 grain Gold Dots. The TNT loads were intended for break-in, practice, and getting a basic sight-in before moving on to the 115 grain loads intended for deer hunting.

 
Trouble: stoppage due to short stroking
However, a problem soon reared its ugly head; misfeeds caused by short stroking. In the AR world, this condition is known as being “undergassed”. To check whether it was due to weak handloads, I tested for correct lockback by loading a single factory American Eagle 115 gr FMJ into a magazine, firing, then checking to see if the bolt traveled far enough back to lock the action open (tested multiple times). The result was much less than 100%.

If you noticed in the specs, this upper came equipped with an adjustable low profile gas block. The upper was shipped with a long skinny Allen wrench to allow adjustment of the gas screw located at the forward end of the gas block. So I started messing around with the gas adjustment. I finally loosened it to the point that it ejected forward onto the desert floor never to be found again! I never did achieve reliable lockback before I lost the adjustment screw. My range day was done.

Okay. I decided that the adjustable gas block was of no benefit to me. At full open it did not resolve my problem, so I placed an order for a standard non-adjustable low profile gas block. I was now going to learn how to replace a gas block.

I struggled with this problem for the next couple of months. I tried wet lubing the bolt carrier, different magazines, a low power buffer spring. The buffer itself was a carbine buffer which was as light as you could go. I gave myself a real education in AR-15 gas system tuning.

Finally, I decided that my next step was to do the irreversible. In reading the reviews for my 6.8 upper on the PSA website, I saw that another guy had the same problem. He said that he drilled the port and fixed the problem. That gave me the courage to give it a try. But what size? I took the upper apart again and found that by testing with various drill bits, the hole appeared to be 5/64”, or .078”. Then I dug around on the web and found the chart below. My barrel is 18” long, which is not in the chart. But .086” is listed as max size for a 16” barrel, and the min size for a 20” barrel, so I figured it has to be “just right” for an 18” barrel. .086” is also known as a #44 drill bit. I ordered one on the web and waited for it to arrive.

 
AR-15 Gas Port Sizing

When it did, I chucked it into my drill, held my breath, and did the deed. I used a drop of oil, and it was swift and painless. Whew! Now would it work?

In my education on AR-15 gas system tuning, I ran into the image below. It shows where your shells should be ejecting based on the tuning of your gas system:

Perfect is 90° ejection. Overgassed ejects forward, undergassed ejects to your rear. With some of my 115 grain handloads, I was now seeing 90° ejection. But with most of my handloads, particularly my 90 grain handloads, I was still seeing undergassed ejection. Nevertheless, my short-stroke stoppage problem seemed to be cured!

Mind-you, this is still with the weaker buffer spring installed. I may still be undergassed, but now at least on the good side of reliable operation. I am hesitant to go to the next size larger in gas port size, but maybe I need to. If I then ended up being overgassed, there are things I could do, like go back to the standard buffer spring, or a heavier buffer, or go back to an adjustable gas block.

Here's another point to consider: I tend to be a conservative handloader. None of my loads so far have exceeded maximum book charges. Could it be that I am undergassed simply because I have a Spec II chamber? It could be that my loads are simply too low-pressure in that chamber. Maybe it’s time to start pushing past maximum book charges to see what the SPC II is really capable of.

Load Testing for Deer Hunting
I reached this point still before deer season. Now that I had reliable operation, it was time to fine tune my 115 grain handloads to get a load that I was comfortable hunting with. But my problems were not yet over.

This rifle was not compatible with my favorite one-piece mechanical gun rest. My rest is made for traditional rifles and would not allow the fore end to nestle down on the front bag because of the protruding pistol grip. I needed an AR compatible gun rest to complete my load testing. I got one of these:

The fore end is clamped into a vice-like arrangement and elevation is adjusted with a twist knob that raises or lowers the platform that the pistol grip rests on.

I just could not get acceptable groups in my load testing using the 115 grain Speer Gold Dots using various charge levels of various powders.

Another well-recommended bullet for game is the 95 grain Barnes TTSX. Well, that ought to do the trick! I’ve always had stellar accuracy results using Barnes Triple-Shock X-Bullets in other rifles. Also, being a lighter bullet than the Gold Dots, velocity should also be great. Nope. I was still seeing group sizes of around 2½ inches at 100 yards with these as well.

Was I having scope problems? The scope I purchased for the rifle was a Vortex 2-7x32mm Crossfire II. For game hunting, I knew that the 6.8 SPC was only a medium range cartridge. I figured that I did not need any higher magnification than 7 power. Also, in keeping with the lighter is better theme, I wanted to go with a physically smaller scope, hence the decision for a 32mm objective lens size.

I had never used a Vortex scope before, but I was impressed when I looked through the Vortex scopes some of you guys had mounted on your varmint rifles at club matches. However, the Crossfire II is not one of Vortex’s premium scopes, in fact it’s one of their most affordable.

I mounted the scope using a Warne X-Skel one-piece AR scope mount. However, when mounted directly to the rifle’s flat top rail, the scope was a still a little too low for my face dimensions. I needed to raise it a little higher. I popped down to Sportsman’s Warehouse to see if they had a “riser for a riser”, as I told the guy behind the counter. They did. They had a Wheeler Engineering Picatinny riser that added another .7” to scope height. Hmm. This seemed a little too tall, but I went with it.

So after I was not able to achieve acceptable grouping, I started wondering about the scope and the tall mount chain. To take the Vortex out of the equation, I went to my scope closet and grabbed a Weaver 3-9x40mm scope that I had recently removed from my .270 Win. when I upgraded the scope on that rifle to a Burris Veracity. I mounted that to the Wheeler pic riser with standard low height rings, leaving the Warne X-Skel mount on the Vortex. This was again a little too short for my face, but it was acceptable.

Back out to the desert for more testing. I was still not able to get the grouping performance that I wanted to see out of the rifle. Deer season was almost here, and I was getting real pissed off at this rifle. I decided that it was time to put it away for now and use my .270 Win X-Bolt for the hunt which I had ultimate confidence in.

Finishing the Rifle
Sometime during all these tribulations, I decided that sharing the lower with my other PSA upper was not what I wanted to do in the long run. I wanted to complete the rifle by putting together a lower of its own.

I found myself up at J&G Sales in Prescott where I started to lay my cash down for a stripped lower by Aero Precision. But then I spotted a complete lower that they were selling, up on the pegboard behind the salesman. “Hey, what’s that?” He pulled it down and showed me.

It was a Ruger AR-Lower Elite. Ruger sells this as a standalone “gun” (since it is the serialized part) for us AR builders. Topping things off, it features Ruger’s excellent two-stage trigger system, the Elite 452. I am a big fan of two-stage triggers. The 4-5-2 nomenclature translates to 4.5 pound trigger pull, 2-stage. In real life, the break comes in at a bit under 4 pounds after taking up the very light first stage. The break is very crisp and predictable.

The lower also features a Magpul MOE grip and 6-position buttstock. This stock has some sort of an internal tensioning system that eliminates all rattle. It’s actually a little stiff to move due to this. Awesome, I’ll take one.

Javelina Season
The deer season came and went. I wrote about that hunt last December in this newsletter, in a story titled “Ben’s First Buck”. The election came and went, the holidays came and went. Before I knew it, my general season javelina hunt was creeping up on me. Since I had put it away, I had not thought much more about the AR-6.8.

I was planning to use another of my ARs, my budget Del-Ton Sport on which I had recently mounted a 3x prism sight. I remain leery about using a little .22 caliber bullet on “big” game, but I knew a guy who used to go after piggies every year with a .22 Magnum, and he swore by it.

But then, about two weeks before the hunt, it dawned on me that I had never done any testing with the 90 grain Gold Dots. I had been concentrating on getting it ready for deer hunting with the 115 grain Gold Dots. The 90 grainers should be perfect for the little piggies, and much better than any .22 caliber bullet out of the Del-Ton.

I threw together a batch of ammo using the 90 grain Gold Dots and just went with the maximum book charge using Alliant Reloder 7. Of the powders listed for this bullet, Reloder 7 was near the bottom for velocity, but it was what I had on hand.

It was time to go back out to the desert for another range test. I set up a target at 100 yards, took two shots and this is what I saw:

The red square is one inch, the white squares are half-inch. No adjustment was needed to the last zero setting I had used. Looks good to me!

From there I proceeded to have some fun popping 12 ounce plastic soda bottles at 100 yards, reliably hitting them. But there was one major difference from my last sight-in session.

Since nothing else I tried to cure the accuracy problem seemed to be working, I started to wonder about that new AR gun rest. One problem that I had with it was that no matter how hard I tried to clamp the fore-end with the vice, the recoil of the 6.8 would cause the fore-end to rotate in the vice. Periodically, I would have to loosen the vice, reset the fore-end to rest flat again, then re-tighten. I began to doubt that a tightly clamped fore-end was a good way to rest the gun for accuracy testing. This time I used bags. They weren’t the best rock-steady bags around, but still they allowed the rifle to more freely recoil.

Ignore the LabRadar chronograph in the photo for right now. It will likely be the subject of some future story. However, it did allow me to measure speed which clocked in at an average velocity of 2832 fps for this load. So here is the data for that, zeroed for 100 yards:

As you can see, the light, short, low B.C. 90 grain bullet won’t hold on to 1000 ft-lbs. all the way out to 200 yards. But a javelina is a much smaller animal than a deer, so it doesn’t need as much power. This rifle was finally ready to go hunting!

In the Field
When the javelina hunt finally came up, I decided to take two rifles along as I normally do. You just never know if something will go wrong with one, and you will need to rely on the other. Of course, the AR-6.8 was one of them and my Howa bolt action in 7.62x39 was the other. My intent was to trade-off, one day hunting with one, the next day hunting with the other. But it didn’t work out that way.

I hunted with the Howa for only one day. In my story about that rifle, I mentioned its one major weakness. The way that the detachable magazine is retained leaves it highly vulnerable to being accidentally released. While walking in the field, I accidentally released the magazine just carrying the rifle over my shoulder. I ended up walking with a round in the chamber, but the magazine in a pocket to avoid the chance of dropping the mag unnoticed. This is not good if action needing multiple shots should suddenly be required.

By contrast, I was totally delighted by the way the PSA/Ruger AR-6.8 carried. Since it is a dedicated hunting AR, I only have three 10-round magazines for it, no hi-caps. Hi-caps would only be in the way on a hunting rifle.

For the most part, the rifle hung well on my shoulder. It didn’t try to keep falling off. When carried in the hand, the balance point is right at the front of the magazine well. The total field weight scoped, with sling, and the magazine loaded with 5 rounds, ended up right at 8½ pounds. Lighter would have been nicer, but that’s not bad. That’s a good all-up field weight for any hunting rifle.

The biggest problem I had with field carry of this gun was on those cold mornings. All metal construction (okay, except for the Magpul parts) transmits the cold right into your hands. I could feel the cold right through my Mechanix Wear camo gloves. It was best to leave the gun over my shoulder until the day warmed up a bit.

Man, I really wanted to find those piggies to finally take a game animal with an AR, but it was not to be. I hunted for 5 days straight without seeing any. Anyway, it’s good to get out in the field with a new rifle. Maybe next season.

In the meantime, I will try again with the heavier bullets to see if I can get the gun to shoot those with accuracy. At this point, I really believe that the accuracy problem I was having lies with the rest. Plus, I will try to cautiously go past maximum book loads to test functional reliability, ejection pattern, and velocity. Maybe I will also mess around a little more to optimize the scope setup.

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