Building An Accurate AR15

December 13, 2019 – John Snell

This story starts with a friend wanting to use an AR platform to shoot ground squirrels, and me trying to ‘guild the Lilly’ (what’s new there?)

In the last few years we have been told that we can buy an accurate MSR (Modern Sporting Rifle) from several commercial builders, who will naturally charge for their experience, time and materials. In general, the pitch is that you can expect sub MOA (Minute of Angle) when using ‘quality’ or ‘match’ ammunition. Apparently these builders are satisfying their customers, as you never hear of any going out of business.

So my friend purchased a very nice Rock River rifle, and I purchased a STAG Varmint upper to use with either my DPMS or Ruger lowers. Since I had already put a better trigger in the Ruger, its been the home for the uppers I’ll be telling you about in this story.

My friend’s rifle shot about 1.5 MOA with several commercial and military surplus loads. For him this was adequate because he says its more accurate than he can shoot. In my case, having been ground squirrel shooting for several years now, I know I will get the best experience from a rifle that can reliably hit the squirrels vital zone (1”x1-1/2”) at 500 yards or more. My Savage 12 FCP in 204 Ruger has been shooting 0.2 to 0.3 MOA groups ever since I finished load development, it takes an expensive and heavy for caliber 40 grain Berger bullet, turned case necks, neck sizing and loading with hand dies to consistently achieve that level of precision. I want at least 0.5 MOA from my MSR or it won’t be much use on the tiny targets. In addition, I want to use the thousands of 5.56 NATO cases I have (mostly Lake City), and I want to load them on a progressive press.

PRESSES

Back in the stone age (1980) I was doing a lot of competitive pistol shooting in the NRA Handgun Silhouette discipline. At that time Hornady’s “Projector” progressive press was my choice, but I was paying attention to Dillon and using their powder measure. That Hornady press worked – right up until it crushed the primer delivery system. Attempts to fix the actuator and transport never were adequate, and at about the same time, I found myself 3,000 miles away and work/life didn’t permit shooting or loading. A decade or two slipped by in the blink of an eye, and I finally had things organized so I could consider shooting again. I dug out the Hornady press, and sure enough, it still didn’t work. I contacted Hornady, but by now they had no parts for the old press, and their new press didn’t even accept any of the parts that might reasonably be expected to interchange. Bah-humbug! Again I looked at Dillon. They still had the exact same press(es) as a couple decades before, plus some new ‘stuff’. Their customer service received consistenly top-notch reviews. But before I bought a new progressive press, I dug out my RCBS Rock Chucker that I had purchased in 1966. That press loaded all the rifle ammo I shot for several years, and between work and family, I turned to other pursuits. Fast forward to 2013, the year I retired. I was enthralled with shooting the abundant Piyute Ground Squirrles that inhabit Southwestern Idaho. The opportunity to shoot them played a large role in choosing this area to retire to. Once again the presses were resurrected from storage. And once again, the RCBS Rock Chucker was instantly ready to go to work. The Hornady remained out of service, and even eBay was no help!

A friend let me shoot his Savage 12 FCP in 204 Ruger. I watched squirrels vaporize at 100, 200, 300, and sucumb dramatically at 400 and 500 yards – I was smitten! So, I immediately purchased my own Savage 12 FCP and went to work on load development. Just before retiring I comissioned Russ Haydon to build a 308 varmint rifle using a new Remington 700 action. This was the last rifle Russ built before he retired, and it is a tribute to his years of bench rest competition. He blueprinted the action, chambered and installed a Shilen barrel, and bedded it in a thumbhole stock I provided. The rifle shot in the 0.3 MOA range from the very first shot. When Russ and I were wrapping up the 308, he introduced me to Wilson Hand Dies which use an arbor press (see the notebook). He also introduced me to neck turning, so I came away with a K&M lathe and all the bits and pieces necessary to turn necks accurately. As I was trying to get the 204 into the sub-half minute range, I purchased Wilson Hand Dies for it and added 204 components to the K&M lathe. Between the two I finally got the rifle consistently into the 0.25 MOA range. Using hand dies while satisfying and actually somewhat faster than conventional dies in the Rock Chucker, just wasn’t going to keep up with the ammunition volume demanded by the squirrels. It was about this point where I realized I could use a 17HMR on short range squirrels and save what remained of the 204’s barrel life on 300 yard plus shots. It was a year or two later, my friend who has a ‘prepper’ approach to rifles talked me into the AR platform for squirrels, it wasn’t a hard sell.

Ok, now I’ve got you caught up as to what and why I am interested in for the AR. Let me finish up about presses.

To satisfy the high demand I knew we would have for the AR’s where squirrels were concerned, I suggested to my friend that we split the cost of a progressive press. This time I chose Dillon, as I should have back in 1980. The XL650 with automatic case feeder assembled quickly, and started producing good quality ammo immediately. In the course of searching for accuracy for the AR’s I upgraded from Dillon Steel dies to Redding Type ‘S’ bushing full length sizer and Redding Competition seating die. Concentricity has improved significantly, and the loads I make on the XL650 now show less than 0.005” runout at the bullet tip.

My advise to any of you who are thinking about getting a progressive press, is to give Dillon a hard look. I’m very happy with mine, its easy to setup correctly, easy to use, and easy to convert between caliber be they rifle or pistol.

LOWERS

For the ‘Accurate AR’ project, I wanted to make use of the MSR platform’s configurability. I chose to use my Ruger SR556 (piston) rifle’s lower and upgrade as necessary or desirable. The lower already had an upgrade Ruger trigger, Magpul STR stock, Ruger enhanced grip, Magpul MOE trigger guard, and Magpul B.A.D lever. The B.A.D lever is possibly the best money spent on this lower. It allow you to release the bolt using the tip of your trigger finger. No breaking cheek weld! Initially my goal for upgrading this lower was to create the best SHTF combat lower for my personal use.

In order to re-purpose this lower to achieve best accuracy I have added these accuracy enhancing options.

Triggers

I replaced the Ruger Enhanced trigger with a Trigger Tech ‘Adaptable AR Primary Trigger’ https://triggertech.com it is adjustable.  I’ve run a test on the trigger pull weights of several of my firearms and posted the results here.  I set the trigger to 2lbs initially, and doubled on several squirrels when I didn’t want to, so I’ve chosen to set this trigger right at 3lbs.  I really like this trigger!  Very crisp break and no creep and very little overtravel.  I would not recommend it for CQB, but for deliberate shooting I know of nothing better.

I learned to shoot competitavely at the tender age of seven using a Mossberg target rifle. As I grew the next couple of years, I moved up to the Remington 40x rifles our US Army supported club owned. The Remington 40X has a legendary trigger. At the time, I thought it was the best there was, and I got really, really good at NRA Smallbore with that rifle. In fact I got so good, that I was winning state championships. The thinking went that I should have my own target rifle, and that since the Olympics were a goal, I should have the rifle most often used in the Olympics, which at the time was Anschutz’s 1413 International. Don’t get me wrong, I LOVED the Anschutz, but I always could shoot just a fraction better with the Remmington 40X, and I believe it was the trigger that made the difference.

In the years since, I’ve shot every crappy trigger I’ve been handed better than anyone else could. It sort of got to a point where it didn’t seem to matter, I had an ‘educated’ trigger finger. Shooting big Smith and Wesson revolvers in double action (just because I could) at NRA Regional Shilouette Matches and so on.

This business of wringing sub-moa out of an AR has turned my world upside down!

In analysing why it is that I continue to be stuck at 1-1/2 to 2 MOA I’ve come to the painful realization that after the trigger breaks, if there is trigger over-travel, if there is excessive lock time, and if there is an opportunity for the relationship between the lower and upper to change, that it may be possible for the mechanical systems of the rifle to be the cause of the large dispersion.

Think about it this way; you hold the rifle in four places. The forend (resting on a bipod), the cheek weld, the pistol grip, and your shoulder. Of these, 3 out of 4 are on the lower receiver. You aim the rifle using sights attached to the upper. When the trigger releases the hammer it takes about 10 mS (10/1,000 Seconds) to travel from its cocked position to its fired position, hit the firing pin and detonate the primer. From that moment it takes about 750 uS (750/1,000,000 Seconds) for the bullet to exit the barrel.

To move the point of impact 1” at 100 yards (ok 1.04”) requires 1 MOA of change in the direction the barrel is pointed. If we use a 16.25” barrel and assume the chamber is the ‘point of the triangle’ then the muzzle would have to be displaced by 0.005”. To give this meaning, a sheet of printer paper is typically 0.004”

The hammer being released causes an equal and opposite reaction which may move the lower, or both the lower and upper. Then the impact of the hammer hitting the firing pin causes an opposite reaction again maybe moving the lower or both the lower and upper.  There is also the issue of shooter induced movement due to trigger pull technique and rifle holding technique.  The acceleration of the bullet then causes an equal and opposite reaction (recoil) which definitely moves the upper, and may move the lower. The recoil motion begins the moment the bullet begins to accelerate, and continues to increase in velocity until the bullet exits the barrel, then the gas jet pushes the upper some more, but at this point the bullet is no longer under our control.

It is all that motion that we attempt to control with our hold, our posture, trigger technique etc. and the mechanical configuration of the rifle.

Long range shooters will tell you that ‘torque’ on the trigger is definable in the groups shot at long range. They never mention how to make that torque uniform, instead they eliminate it by moving the thumb of their firing hand to the same side of the stock as their trigger finger, and pulling as straight back as possible. In other words, they don’t grasp the stock with their thumb. Most also refrain from grasping with their fingers (for the same reason). If we follow this advise, we now only have contact/control of the rifle at three places (forend, cheek, and butt).

Shims

To keep the relationship between the hammer and the firing pin consistent, to keep the point of aim consistent, and to remove all doubt … I have installed a ‘National Match Shim’ from Wheel Diamond http://www.wheeldiamond.com/nationalmatchshim.htm.  This little ‘U’ shaped piece of metal starts out thicker than the distance between the upper and lower receiver in the area just above and behind the rear takedown pin.  To install it, one spends a lot of time slowly removing metal using a jig supplied with each wedge and your favorite grade of grit.  When correctly installed, the wedge exactly fills the space such that the takedown pin requires a bit of pressure to install, and might require a punch to deinstall.  With that pressure distributed fore-aft and side-to-side, the wedge locks the upper and lower together with zero rattle in any direction, and removes any chance inaccuracy originates with a sloppy upper-lower fit.

There are other ways to accomplish similar tension, the most popular I’ve seen involves a set screw that acts through the grip screw to apply tension.  I haven’t tried one of those, and I may be incorrect in assuming that they continue to allow a slight amount of side-to-side wobble.  I can however tell you that very minute changes in shim thickness (on the order of 0.00005″ – yeah, I meant 50 millionths) can be felt as a change in tension on the takedown pin’s insertion and extraction force.  I think the screw approach might work best if you are prepared to unscrew it before removing the takedown pin.

UPPERS

There isn’t a whole lot at an upper offers in the accuracy department.

The only ‘adjustment’ I am aware of is squaring the front surface of the barrel mortise so that the barrel extension flange seats square and has better than 75% contact when the barrel nut is torqued.

There are two ways to accomplish this.

Glue the barrel extension tenon into the barrel mortise using a gap filling high temperature, stable glue.  The consensus is to use Locktite 620 (green).  To be successful, the application should include enough 620 on the barrel extension tenon to completely fill any gaps.  The barrel should be rotated as it is inserted to evenly spread the 620 inside the tenon/mortise junction.  At the same time there needs to be enough 620 on the barrel flange to fill any gap between it and the front of the upper receiver.  Fortunately, Locktite 620 has a long ‘open’ time, and can be cleaned up with paper towels (don’t use any solvents) before it sets.  620 will not harden except inside spaces smaller than about 0.020″, so squeeze out is easy to remove.  Once the 620 is installed, torque the barrel to at least 50 ft-lb to ensure the flange is pulled up tight.  Be careful to ensure the gas tube clears the barrel nut with no contact.

The second method is to lap the front of the upper receiver using a tool with a square face that fits inside the bolt raceway with little slop.  Such a tool is available from Wheeler Tools, or if you have a lathe you can easily turn one.  My Wheeler tool has a 5-1/2 inch long 0.997″ diameter section to fit the bolt raceway, a 1-1/4″ long 0.750″ section to prevent lapping compound from entering the bolt raceway, and a 1-1/4″ x 1.475″ section which establishes the lapping surface on its interior face.  After a few uses, I true up the lapping face, if you buy this tool, and you have a lathe you may want to start by truing the lapping face.  If not, don’t worry, the lapping action still produces a square face on the receiver even if the lapping face is not running true.  When you remove material from the receiver front, you move the position of the barrel flange toward the rear, and thus move the point at which the barrel nut will stop at a given torque.  If you are using a barrel nut with holes for the gas tube, you may find this useful for achieving your desired torque and doing so without backing the barrel nut to the previous gas tube hole.

On my STAG-15 barrel I used the LockTite method.  The barrel didn’t produce the level of accuracy I wanted, and I ended up removing the barrel.  In order to do so, you heat the upper receiver with a propane torch until it gets over 550° F to release the 620.  When I did this, the 620 did not adhere to the barrel mortise, but it did adhere to the barrel extension tenon.  I was able to clean up the interior of the mortise with paper towels while the upper receiver was still hot (probably over 300° F), but the 620 on the extension was very stubborn, and would not come off until I burned it off with the propane torch.  In doing so I got the barrel extension up to ‘light straw’ color, while removing the 620 with a steel wire brush.  I was concerned that the high temperature might have compromised the barrel extension (at this point I was going to discard the barrel), talking to an experienced armourer I came to the understanding that the barrel and extension were not damaged.

The pattern of the LocTite 620 on the barrel extension after being removed showed that I had enough coverage to support the barrel completely around it’s circumference, but there were some gaps I didn’t like.  I had applied the 620 a bit sparingly because at the time I was not sure squeeze out could be cleaned up easily.

STAG

My initial education on building accurate AR rifles included watching a lot of YouTube videos and reading a lot of articles written by gunsmiths who specialize in National Match rifles.  Many either go through elaborate selection  to find barrel extension to upper tenon interference fits, or they use Locktite 620 (green) to glue the barrel to the action.  Needless to say I thought stabilizing that joint was worth a bottle of Locktite.

The STAG upper was shot using MIL-SPEC and commercial match ammo in 55 and 68 grain loads.  Accuracy was about 1-1/2 to 2 MOA.

I then applied the Locktite and … accuracy remained the same. 

My STAG upper simply was not shooting below 2 MOA reliably with anything from MILSURP to Hornady and Black Hills MATCH ammo, nor was I able to reduce group size by hand loading. Incidentally muzzle velocity variation (ES) was typically 50-150 fps with CFE223 and 55 grain bullets. CFE-223 with heavier bullets (up to 70 grains) typically had lower ES of about 30 fps. The few VARGET loads I tested had Muzzle Velocity Extreme Spreads (ES) of between 20-30 fps, and the same was true for BL-C2 loads with 55 and 69 grain bullets. Black Hills 77 OTM ES was 29 for 5 shots.  

I got so upset with the STAG upper that I made a CERROSAFE cast of the chamber, leade and a little bit of the rifling.  On the best looking cast, I believe I can see a different length between the transition from the case neck to the rifling.  It looks like the rifling is longer toward the case neck on one side than the other.  I contacted STAG customer support, explained the situation and they paid to ship the upper back to the factory for testing.  A couple of weeks later, I got the upper back with a note that said they shot it.  Nothing else.  So I called.  The Customer Service rep I spoke to initially had fired the upper at a local range.  When I asked it shot ‘acceptably’ for a 24″ varmint barrel, his response was that he wasn’t all that good a shot, but the upper worked.  STAG declined to do anything else because I had modified the upper by using LocTite, a situation I had carefully explained BEFORE I sent the upper to them.  So BIG WASTE OF TIME – but it was my fault.  Should have sent it back before I tried to correct it.

I put the STAG barrel away thinking I would never use it, and started looking for a replacement.

Some time after I had completed a new upper using the STAG upper receiver, I acquired a Teslong flexible bore camera.  In the course of learning to use it, and interpreting the images it produces, I dug out the STAG barrel to see if I could get a better look at the ‘off center chamber’ problem.  As it turns out, there is a very noticeable difference in the length of the rifling in the chamber, but it doesn’t look as terrible as the casting made it look.

Here’s a look at this chamber using the Teslong bore scope.

Here’s the cerrosafe casting of the STAG-15 5.56 NATO chamber.

STAG 5.56 NATO Chamber Leade #1
STAG 5.56 NATO Chamber Leade #2
STAG 5.56 NATO Chamber Leade #3
STAG 5.56 NATO Chamber Leade #4
STAG 5.56 NATO Chamber Leade #5
STAG 5.56 NATO Chamber Leade #ASTAG 5.56 NATO Chamber Leade #B

This is where things get a little out of sequence, I had completed the PROOF RESEARCH upper to replace the STAG, and wasn’t happy with the results, so I started to build a WHITE OAK upper, but in the course of acquiring the parts, I realized I could install the STAG into the upper I was building for the WHITE OAK and now that I have a better trigger, stock, scope mount it might be the STAG would shoot better.

As of August 2020 that’s where it stands, I have decided that all new uppers will have the same furniture, so I acquired a SEEKINS MCSRV2 Rail System and SUPERLATIVE ARMS adjustable gas block.  In addition, I found an AR STONER upper at MidwayUSA which is made without the forward assist (not even a protrusion for it!) and does not have a dust cover installed.  The cost of this upper receiver is about 1/2 the usual cost, and after building an upper on it, I’m convinced it’s quality is every bit the equal of the MIL-SPEC uppers costing twice as much.  It is now my “Standard Upper”.

The hardware was to be installed with a WHITE OAK National Match barrel, but here I had a STAG 24″ 1:8 Heavy Varmint barrel, that might shoot.  I started to put it together, and quickly discovered the STAG barrel is larger diameter behind the gas port than the standard 1.000″.  I contacted SEEKINS who provided a spare barrel nut, and then opened the ID of the first barrel nut to an ID of 1.052″  which allowed it to clear the barrel.  It also made the barrel nut very thin!  However I’ve torqued it to 50 ft-lb and fired some NATO pressure loads, and as far as I can see the nut is performing as designed.

Barrels

I sent an email around to all the manufacturers of accurate AR barrels that I could find.  In the email I asked how they install their chambers.  Apparently STAG just shoots them in with a CNC. Only Proof Research came back with a complete answer that satisfied when they said “We install chambers by indicating on center and use a piloted reamer.” Presently I found a Proof Research barrel at a discount on MidwayUSA, a 16.25″ 1:8 REECE.

Accessories

Handguards

Gas Blocks

Bipods

Buttstocks

Triggers

Sights

Scopes