The AR-10 Build in Mammoth Sniper Challenge

By Christopher Roberts

It was a cool, partly cloudy January morning when my partner Chris and I had just navigated a 30-foot tower and engaged one minute of angle (1 MOA) targets out to 500 yards. Since Chris was running a LaRue Tactical OBR chambered in .308, these targets were well within his grasp. After his engagements, we ran back down the narrow tower stairs to a dirt berm, where I had only a few minutes to engage multiple targets out to distances of 1000 yards. I was using an AR-10, built in my garage, chambered in the formidable 6.5 Creedmoor. We ranged my first target at 736 yards, I dialed on 5.1 mils. Chris told me to hold left edge of target, and I broke the shot. Ringing steel echoed out across the range, assuring a solid hit. Immediately, I transitioned to my next target, roughly 50 yards further. I held a little high and sent a round downrange, impacting just off the right edge. Thanks to my Leupold Mark 4 ER/T equipped with Horus 58 reticle, I could see splash. After a shot correction, I quickly sent a second round downrange; impact!

Shortly after, we packed up our kit and began our final foot march. In the course of two days we covered 35 miles. In the end, we took 11th out of 43 teams in the tough man division of the 2018 Mammoth Sniper Challenge. I was elated to have my first shooting competition under my belt.

Mammoth is the type of competition where physical endurance and the extreme edge of precision meet. Joe Harris, the match coordinator, likes to put you in awkward positions and then give you slivers of a target to shoot at. What made this entire situation more complicated was the fact that 4 weeks before the competition I had no rifle. I knew that we would have to engage multiple targets rapidly, so, in my mind, the choice was easy; I would build a 6.5 Creedmoor AR-10 as the primary shooter. With its proven track record in respects to accuracy, it seemed to be the natural choice. Unbeknownst to me was just how difficult a build of that nature could be.

AR-10 builds aren’t unusually tough. But, unlike AR-15 builds, the AR-10 has had three major variants: ArmaLite (now known as Armalite), DPMS Panther Arms and Rock River Arms. Eugene Stoner designed the first variant of the AR-10, when he worked as Chief Design Engineer for ArmaLite Inc., ca. 1955. Since then many variants and copycats have emerged, resulting in a wide variety of parts that are not all interoperable.

Not one to shy away from a challenge, and valuing accuracy, low recoil and ammunition availability, I chose 6.5 Creedmoor as my caliber. While the 6.5 has gained enormous support in the last 10 years or so, 6.5 AR-10’s are a relatively new trend. Now I’m not going to sit here and lead you to believe that I’m the first person to try this endeavor. There are forums and Facebook pages with endless information on every build imaginable. What I will tell you is that there were few complete builds published. There were plenty of guys like me, running into problem after problem making their gun run right.

4 Weeks until Mammoth

Before I started my build, I reached out to Chad Hulsizer, a former Army sniper, good friend and owner of Trigg Shooters, LLC. Chad retired from the Army a few years ago and opened his gun store in Cadiz, Kentucky. While stationed at Fort Campbell I’d make what seemed like weekly trips up to his store to buy new guns and pilfer cold Coke’s from his mini-fridge. With his years of experience serving overseas and working in the industry, it seemed like a no brainer to look to him for advice.

A few hours later and a swipe of the credit card, I had a plan in place and parts in the mail. I started with the new ZEV Technologies receiver set and matching handguard. ZEV has made a name for itself in the custom Glock world, and since its merger with Mega Arms, has been making waves with ARs too. ZEV followed the popular SR-25 pattern for its receivers but added a few innovations. First, the integrated ambidextrous bolt release and magazine catch are almost invaluable to a precision shooter. While the average shooter isn’t going to gain a lot from these additions, being able to operate the majority of your rifle’s functions with one hand is huge when speed and accuracy are on the line. Commonly, your free hand is being used to support the back end of the rifle. Second, ZEV chose to use threaded bolts rather than roll pins for installing the bolt catch. Anyone who has ever tried to install these small roll pins into an AR has run the gambit on gouging their lower receiver. Next, ZEV added a roll pin to the upper receiver where the charging handle latch engages. I’ve yet to have this area on a receiver wear out, but I appreciate the innovation. Lastly, they added a small hole on the front of the upper receiver to allow proper alignment with their Wedge Lock handguard.

For the barrel, I have to be honest; it was as much chance as anything else. I probably spent a whole day of my life combing the Internet for a perfect fit but couldn’t come to a decision. Precision was the goal; budget and time were the limiting factors. There are many quality barrel manufacturers out there, but most have a slow turnaround time. Time was tight to build a rifle, check it for function, test ammo and gather data. It was looking bleak when I finally came across a lightly used Ballistic Advantage Premium 6.5 Creedmoor barrel with a 22-inch heavy profile. I had read many good reviews on the products coming out of Ballistic Advantage, and seeing how this barrel was available locally I decided to take a chance. Boy was I glad I did. The condition was immaculate, and the groupings it would come to hold continue to impress. If it seems like it was too good to be true, it was. For all its qualities there was one single problem: clearance. Since the profile of the barrel was so thick, the low profile adjustable gas block wouldn’t clear the handguard. But, it was nothing I couldn’t handle. A little grinding on the bottom of the block and boom—clearance.

When it came to the scope, it seems Lady Luck was on my side again. I had experience with Leupold Mark 4 scopes on rifles at work and recently began to master the Horus 58 reticle. So, when I found a new Leupold Mark 4 ER/T with Mil turrets, I jumped. The H58™ is the combination of a tactical milling reticle and a hold-over Christmas-tree-type system. With this reticle you can achieve quick hold-over adjustment assuming you’re able to see splash from your round impacting near the target. When you first transition from the simpler Leupold Tactical Milling Reticle (TMR™), the H58 seems cluttered. But, with some range time, it becomes clear how intuitive this system is. The H58 hold-over reticle would prove to be invaluable in the upcoming competition.

Now, Chris is one of the most intelligent shooters I’ve ever worked with, and part of the reason is his constant pursuit of the latest technology. When he told me that he had a new anti-cant device to try out, I was a little skeptical. I’ve used these devices before and wasn’t too impressed. Usually, they involve a bubble level that floats inside of a glass bulb. Simple, but not very precise. In a game like long-range shooting every degree of error counts, not to mention they are often hard to see if you’re behind glass. Enter Long Range Arms (LRA) and their Send iT-XSL. LRA claims that you’ll, “never have to take your eyes off the reticle” because, “the bright LED lights can be easily seen out of your peripheral vision.” LRA further states, “The Send iT-XSL level is 3-5 seconds quicker and far more accurate than a mechanical level.” Ok, now they have my attention.

The design of the Send iT-XSL is small with a simply color-coded line of LEDs that clamp horizontally to a standard Picatinny rail on top of your receiver. They also offer a scope clamp that allows the device to be mounted vertically. I chose vertical because I wanted to keep the rifle streamlined for packing in a case. After mounting the Send iT-XSL I flicked it on to try it out. It really is simple to operate with just a rotating knob that controls the intensity of the light. I preferred to keep mine low so the light wasn’t distracting. The LEDs range from red when the rifle is canted one direction, blue when canted the other and green when there’s zero cant induced. So “GO” on green became part of my shot process. This entire concept was easy to work with, requiring little to no extra attention, and increased my hit probability dramatically on the longer distance targets.

To tie up the loose ends, I picked a Seekins Precision Enhanced Builders Kit, Geissele trigger and generic rifle buffer system. I’ve been fond of what Seekins has been doing with their precision rifles and figures; if it was good enough for them, then it’d be good enough for me. The price didn’t hurt either. I also decided on a Geissele Automatics Super Semi-Automatic Enhanced (SSA-E) for my trigger. I’ve been using Geissele triggers in my ARs for years and continue to be impressed by the SSA-E. Similar to its predecessor, the SSA, the “Enhanced” is a crisp, two-staged trigger with a lighter overall pull, weighing in at just 3.5 pounds.

Now for the ammunition. I needed something accessible enough to find in large quantities. I didn’t want to be shooting different lots of ammo during the competition. I estimated I would need roughly 500 rounds for testing and competition. I scoured the Internet and local stores and came up with three main offerings: Hornady ELD-Match in 140gr, American Eagle OTM 140gr and Federal Gold Medal Match 130gr. The Federal and American Eagle offerings patterned well, all Sub-MOA, but not as well as the Hornady. Grouping was at .36 MOA, and I could tell it had more in it, I just didn’t have the ability.

2 Weeks until Mammoth

Rifle was built and off to the range we went. First stop, the CMP Marksmanship Park in Talladega, Alabama. My partner told me about the electronic targets they used, and we decided it would be a great training opportunity. We were right. Not only do the electronic targets save you a lot of leg work walking up and down range, they give you measurements of your groupings. Even better, the CMP Marksmanship Park placed these targets at multiple ranges, from 100 yards out to 600 yards. You might be wondering why this is specifically important. Shooters often quantify the accuracy of their rifle and ammo off of how tight their groupings are at 100 yards. While this is a good place to start, a lot of value can be gained in the knowledge of how your rifle can pattern at ranges greater than 100 yards. In addition to patterning groups, you can get accurate measurements of the effects of wind on the rounds at varying ranges. This information combined with finding your ammunition’s standard deviation, will help you to determine hit probability. In the case of my rifle, I was grouping at .75 MOA at 600 yards with negligible shift due to wind.

This trip wasn’t all rainbows and butterflies. Everything started fine; I was getting bolt lock on last round fired and rounds were ejection at the 3:30-4 o’clock position. No jams or visible marring on the casings, and the primers looked fantastic. Toward the end of our trip I started to experience failures to feed on the second or third round of each string. We checked the gas setting, and it was wide open. Tried new magazines, nothing. Tried new ammunition, and the problem just seemed to get worse.

Two days and a few calls to Chad at Trigg Shooters later, we were headed to another range with three different types of magazines and two types of ammunition. To our dismay, the failure to feed continued. We finally determined that the fail to feed was being caused by the rifle short stroking: a malfunction caused by the bolt not fully cycling to the open position, therefore not being able to grab a new round from the magazine. But my question was why? The gas was all the way open. Obviously, there wasn’t enough pressure to allow the bolt to cycle completely. That got me thinking—what if it wasn’t the gas system? Maybe I had too much resistance coming from my buffer system. With a quick swap of the lower from Chris’ OBR, BINGO! Smooth operation, proper ejection of casings and consistent feeding. Still, why? Well, Chris had a carbine length buffer tube with a heavy buffer. I had a rifle-length buffer tube with who knows what weight buffer and spring.

I did some more research and quickly discovered this was a common problem. Most builders were successful with either an adjustable buffer system or a carbine length buffer system running a heavy buffer, approximately 5 ounces, and a light or standard spring. The why was simple. A heavy buffer would allow the bolt to stay locked longer, building more pressure, while the light spring wouldn’t prevent a full cycle. Now the task of finding a heavy buffer and a carbine length buffer system began. Cursory searches of local shops yielded a shorter buffer system but no luck on the heavy buffer. After a desperate trip to a local gunsmith, I learned about the difference in weight in buffers and the fact that the buffer could be taken apart and weights manually replaced. All those times shaking a buffer with my hand, and I never realized the clinking sound was from individual lead weights inside. Weights that could easily be removed and replaced. But, replaced with what? Tungsten! Tungsten is heavier than lead by volume, therefore taking a 3-ounce buffer to 5 ounces with the simple swap of one weight. A quick Amazon order and I had tungsten weights in the mail.

4 Days Before Mammoth

With a functioning rifle, ammunition to spare and data gathered, it was time for some practice. A quick zero confirmation and we started to work our way farther out on the range. Chris was giving accurate ranges and amazing wind calls. We took the rifle out to 900 meters and back to zero again. Our data was on, and the turret values on the Mark 4 were true. All that was left was to practice alternate firing positions. Check.

Mammoth Sniper Challenge started on a cold, breezy morning in January. After a quick 3-mile ruck, we arrived at the first stage. Time to see what we were made of. Joe Harris and crew put us through the ringer. Rinse and repeat. Stage after stage of incredibly small targets placed at unreal distance, shot from precarious positions. Oh, don’t let me forget, a ruck between every stage.

Later that evening, Chris and I settled into the campsite and realized we woefully underestimated how soul crushing this was going to be. Our laser range finder wouldn’t give accurate readings; every target was a different size, so you couldn’t mil them; and trace was virtually unreadable. Our game plan had to change. New plan—leave every stage with points. Shoot at targets with a high hit probability. Crush the rucks. With teams falling out of the competition every timed ruck, we knew if we could just finish we would have a chance. Chance at winning? No. We just didn’t want to embarrass ourselves.

43 teams started on Day 1. Approximately 22 were able to finish after two days and two nights of bitter cold, rain and brutal stages. We scaled obstacles, crawled through ditches, dragged sleds and carried sandbags. We shot from a tower, A-frame and standing on a bench. Carried rucksacks weighing near 60 pounds for 35 miles and slept under the stars.

Post Mammoth

Would I do anything differently? Absolutely. For starters, I’d lighten up my rifle. When it was all said and done, it weighed in at 15 pounds. I could save 3 pounds just by switching barrels to a carbon fiber offering. Ounces equal pounds, pounds equal pain. Next, my boot choice failed me. I opted to wear comfy hiking boots even though Army boots had carried me on many 12- and 25-mile rucks with no foot problems. In the end I had six blisters. Lastly, we should have brought multiple laser range finders. The most successful team ran with weapon-mounted devices. Having a set-up like that proved invaluable along with having a spare. Being able to range a target with multiple devices could have been a game changer. If you don’t know how far your target is and can’t see trace, you don’t have a chance.

Lessons were learned, and Chris and I became a better team. In the end, we were able to squeak in at 11th place. We vowed to do it again but show up better prepared. The prize table didn’t hurt either.

This article first appeared in Small Arms Review V22N6 (June 2018)
and was posted online on April 20, 2018


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