The Essentials of a Precision AR-15

General Firearms Precision Rifle - Semi-Auto
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The Essentials of a Precision AR-15

Without making this overly complicated, you need three basic components for a semi-automatic AR-15 to produce its best mechanical accuracy (technically, precision): a match-grade barrel, a quality free-float handguard and match-grade ammunition, preferably hand-loads tuned for your barrel. (The free-float handguard doesn’t add to the accuracy of the barrel per se, it simply prevents any outside influence from detracting from the accuracy of the barrel.) Anything after that will not immensely improve the mechanical accuracy of the semi-automatic AR-15, but several things can help you, the shooter, shoot to the level of the intrinsic accuracy of your semi-automatic AR-15.

While there are certainly gifted individuals among us that can do amazing things with iron sights, most of us will benefit from using a quality, high-power scope to achieve the highest level of accuracy from a precision AR-15. In order to hit the exact same spot on the target every time, you have to be able to see that you are holding on the exact same spot every time. It’s also important for the scope to be mounted at the proper height and at the correct eye-relief for the particular scope. One of the most common errors I see with scopes mounted on AR-15s is the scope not being mounted far enough forward for its eye-relief.

In order to maintain that exact hold on the target throughout the trigger pull, it helps not to be fighting with a heavy, gritty, stock trigger. There are a variety of aftermarket triggers now on the market for both standard size trigger pins and the larger Colt trigger pins. Personal preference will definitely play a role in trigger selection. Among the two-stage triggers, I’ve found the Geissele triggers to be the smoothest, lightest, most consistent and most reliable. For single-stage triggers, it’s hard to beat the JP Enterprise Fire Control System. Keep in mind that the JP trigger does require fitting.

Following the scope and trigger selection, some shooters will find that items like aftermarket grips and stocks will help them achieve a better “fit” with their AR-15. (Shooters don’t all come in the exact same size and shape.) Once you’ve put your precision rig together, you have to find a match-grade factory load that your barrel “likes” or better yet, develop a match-grade handload for it.

A semi-automatic AR-15 is not going to shoot as accurately as a precision bolt-gun, but today’s precision AR-15s are capable of a level of accuracy that is truly outstanding for a semi-automatic rifle. The 10-shot group pictured below was fired from one of my Krieger barreled semi-automatic AR-15s from a distance of 100 yards. I used Sierra 55 grain BlitzKings that were hand-loaded on a Dillon XL-650 progressive press. The load was developed using my Accuracy Node Detection Technique. The group has an extreme spread of 0.474”.

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All fact, with supporting data. As usual, great post. Thank you for putting the time, money, and effort into these threads. It’s truly appreciated by many.

It’s also a great reminder, seeing a post like this, both here and in life, to stick with the facts, and not speculate. I find myself guilty sometimes.

Also, great shooting. That is fantastic precision.

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About bolts and carriers:

I’ve been wondering how much a head spaced bolt affects accuracy. Most high quality barrels are matched at factory, but for those of us who didn’t get the memo and ended up without a matching bolt, I was wondering about the following:

1.- A Mil-Spec 5.56 bolt runs between 1.4646+.002 (5.56 Go) to1.4748 (Colt field). MSTN specs say they headspace at 1.4646+.002 at factory. This leaves a potential gap up to.0062 that affects case expansion, if one were to throw in a Mil Spec bolt that is on the lower end of the headspace scale on an SS barrel. Would anyone have real world data on how detrimental this gap can be? What would the minimum headspace required be for a half minute gun?

2.- Even if the headspace issue was solved, wouldn’t the bolt have to be perfectly perpendicular to the barrel? Following the logic that all components should be parallel and perpendicular, (given the case for billet uppers and the existence of Brownel’s lapping tool) would there be a case for using a Young Manufacturing bolt (match or mil-spec) that has increased contact surfaces and trued bolt lugs? Anyone have real world experience with incrementing accuracy by using these BCG’s?

3.- The Military stopped using chrome carriers and bolts in the 60’s. Larue currently uses a chrome bolt. Is there accelerated wear on the barrel extension/upper receiver contact surfaces due to the hardness of chrome lining or is this a myth?

The textbook answer from the internet to these questions seems to be “these variables are all minimal if not negligible, shut up and throw in a BCM carrier”. Wishing to improve upon that, would anyone have any technical insights? Because getting a box of headspace gauges and a box of bolts to try and match 1.4646 sounds expensive and not fun at all.

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My MSTN Upper has an LMT Bolt thats spec’d to 1.4646 + or - .001" and it normally shoots between .5" to .8" 5shot Groups via Mk262 Mod 0/1

This is with a Young N/M Hard Chrome Bolt Carrier where the whole BCG has been IonBond DLC’d, and is all inside a Vltor MUR-1a Upper Receiver. Oh and almost forgot, it’s the Noveske 16" Intermediate-Length Gas 3-Groove 1:8 Twist Stainless BBL.

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HD1911,

That sounds like an awesome rifle. About your carrier: How deep is the N/M’s grove wear in the MUR? Was difficult closing the bolt until it was broken in? Also, are you running that N/M with an H buffer or HD spring with the intermediate tube? Does the Mid-length system have a propensity for bolt bounce or high carrier velocity able to set back the ogive on the 262 Mod 0?

I’m emulating one of the older MSTN rig specs: Garden variety Noveske Recon 1-7, Larue upper & 11” hand guard. Lookin at the N/M +H2+red spring. I’m doing the homework on this thing as I graduated into precision from old-school carbines with classic FSBs.

Appreciate the advice

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I’ve seen posts on here that claim UR - LR fit isn’t important to accuracy.

Personal experience tells me otherwise. A loose fitting UR will wiggle around enough to disrupt your POA, this becomes more important at distance.

I’m not advocating using some gimmick like an accu-wedge. Select your components to fit together tightly to minimize the play in UR / LR mating surfaces.

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HKGuns,

How much POI shift or group size growth are we talking about? And at what distance is this noticeable?

Apparently the goal of this game is to keep a bullet as concentric as possible to its neck, case walls, flash hole, and primer, inside a barrel chamber that is perfectly concentric to it’s crown and outer profile. This barrel must then be as concentric as possible with the inside diameter of the upper, the bolt face, bolt lugs and the carrier group, while maintaining exact perpendicularity with the upper rail to be parallel to the optic.

I agree that after all that tolerance work, it would be a waste to slap that group on a wobbly lower. It makes sense to apply the same high tolerance logic to the relationship between the URG and the lower. However, I noticed that after hammering in the rear lug of a tight URG in into a tight a lower, several hundred rounds of firing makes it settle and a slight play can be felt that could require additional tightening. The only way I see to correct it is to run epoxy bedding like the National match dudes.

Anyone here epoxy bedded a lower before and achieve noticeable results?

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How much POI shift or group size growth are we talking about? And at what distance is this noticeable?

The math shows that a .1 degree amount of slop (up/down or side-side) equates to a shift of ~ 2 inches at 100 yards and gets worse as you increase the distance, obviously.

Real world could be slightly different in that the slop or play may not be consistent from shot to shot given you and I are the largest variable.

Anyone here epoxy bedded a lower before and achieve noticeable results?

I have not.

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You know, I haven’t really even noticed any Groove Wear. I could try and get some pics of the inside of the upper receiver and show you.

She was stiff at first, but always Returned to Battery, no problems. Now she feels like she’s riding on Ball Bearings… the only noise and feel that’s apparent is the Sprinco Green spring moving in the Vltor A5. She’s ran Flawlessly with A5H1 & A5H2 Buffers… no issues at all.

Nothing wrong with the older MSTN Upper you’re mentioning.

Oh, and I’m also actually blown away at how tight the Upper to Lower fit is on this thing… the Vltor MUR-1a is mated to a Noveske Gen2, and there is literally not one single bit of play. Yet, it isn’t a workout nor does it take a special song and dance to get the pins pushed back in.

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I would like to add that getting a very comfortable cheek weld which will help the shooter see consistently through the center of your scope every time. For me I like to rest my cheek bone on top of my rear stock. I read a lot of guys mount their scope as low as possible. I tried the Nikon P-SERIES mounts and found that if I got rings that was about 1/8" higher put my cheek weld perfect for me. Mine are Warne Quick Detach Ultra High Aluminum rings.

http://www.sportsmansguide.com/product/index/nikon-p-series-mounting-system?a=993875

http://www.amazon.com/Warne-Scope-Mounts-Detach-1-Inch/dp/B0002ZPS1O/ref=sr_1_7?ie=UTF8&qid=1436420383&sr=8-7&keywords=warne+quick+detach+rings

The scope actually lines up with the end of the charging handle. Picture is at a bad angle.

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More important, a “precision shooter” behind the rifle.

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What do you think of a DDM4V11 S2W with a NF SHV 4-14x56 MOAR reticle in a NF base as a precision shooter for a beginner? Preliminary shooting indicates it likes 77gr. Nosler bullets. It is new, have not fired it much.

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I found this post by MTSN on Sniper’s Hide. I found it to be very informative.
http://forum.snipershide.info/showthread.php?t=239418

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Do you have any experience with the Aero Precision M4E1 upper? It mounts the handguard directly to the upper, and doesn’t require indexing the barrel nut. I like how it works, but won’t claim to be skilled enough to notice the difference.

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Thank you for sharing. Very informative.

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Interesting and thoughtful discussion - some additional points to bear in mind…
Headspace that varies a few thou either way has virtually zero affect on accuracy. In the context of this discussion, a headspace variance of say +/- .002 isn’t going to show up on the target. So! I would ask those that have the their hand on the BS flag to take a breath and relax for a moment.

Consider this: In every rifle that uses a spring loaded ejector, a chambered cartridge is pushed forward in the chamber by the ejector so that the shoulder is up really close and personal with the shoulder in the chamber. When fired - and assuming the headspace is not so excessive that the firing pin can’t touch off the primer, the case becomes almost plastic and sticks to the chamber walls (by design). This is evidenced by two points;

  1. Excess headspace is shown by primers that are backed out of the case and
  2. Cases will stretch near the back - usually about .300" up from the case head. This is where we would see insipient case head separations.

A barrel / bolt combo that is on the loose side of the headspace spectrum will almost never show any degradation in accuracy. The reality is that at the time of firing, headspace is really not a factor in accuracy. (Keep the BS flag in your pocket!) The reason behind this is again, the ejector spring pressure keeps the case all the way forward in the chamber…always. Now - uneven lug seats, bolt faces that are not perpendicular to the bore, non concentric chambers will all affect accuracy. The assumption through out this discussion is that the rifle has no other flaws / variables.

Conversely, If you ever want to open up headspace - just leave your sizing lube on a case or lubricate your chamber - it will increase bolt thrust so much that after 5 or 6 rounds, you are almost assured of an additional few thous of HS and proabably a sheared lug or two. Clean dry cases and chambers are the waypoints of operator & rifle longevity.

For those circumstances when the headspace is truly excessive - we are now dealing with a serious safety issue. Excess headspace will push those primers out too far and they will leak at best. Worse is a completely blown primer. Catastrophic, is headspace that is large enough to cause a case head separation where the cartridge has stretched so much that it separates upon extraction.

I have in the course of many many years, chambered hundreds of barrels - both in bolt guns and gas guns. For match type rifles - I always headspace to the SAAMI minimum. Not for accuracy, but rather knowing that the vast majority of shooters that want this type of rifle built are also hand loading for it too. Minimum headspace will prevent excess stretching of the cases and prolong their life.

A few other points to ponder as well. I would ask that if you have the capability to accurately measure cartridge headspace - do so for some enlightenment. In factory ammo, it is not uncommon to see match grade ammo have cartridge headspace that is .004 - .006 under SAAMI minimum. This is why virtually every round of OEM ammo will chamber and fire. (A side note to measuring Cartidge HS using the Stony Point / Hornandy or any gauge of that type - proof it first against known / verified HS gauges - I tested some Stony Point examples and they were over by +.008.) Another point to ponder - consider a .45 ACP 1911… striaght wall case, by design, on paper, headspaces on the case mouth… In reality, it actually headspaces on the extractor - the extractor holds the case in relation the bolt/slide face. The chamber could be +.020 and the carrtridge will still fire and function.

Worse case scenarios for any bad headspace conditon is inadequate headspace, coupled with a broken or worn rifle that will fire with the bolt unlocked - almost universally results in significant operator injuries. I did a forensic eval on an M14 NM rifle that fired out of battery. The culprit was a short headspace condition (cartridge based) and a broken feature in the receiver. M14’s have a bridge that connects one side of the rcvr to the other - located near the rear of the bolt. Machined into this bridge is a cam type ramp that retracts the firing pin upon bolt unlocking /extraction. It also has a notch that allows the L shapped firing pin to move forward through the notch only when the bolt is fully rotated and in the locked position. Years of firing stout 1K loads had battered the bridge and a portion of it eventually cracked and literally fell out of the rifle. This eliminated the safety functions - the bolt failed to go into battery due to a long cartridge (improperly sized handload) and the operator - not knowing the bolt was unlocked, pulled the trigger. The rifle fired out of battery. The shooter had glasses on and was peppered with brass and powder. The right bolt lug sheared off and actually went through the visor of his hat…Lucky isn’t a strong enough word.

So - in the end - HS is vitally important - but it isn’t an accuracy issue that some think. I personally want and build all my stuff with SAAMI minimum HS. But I also control every round that goes into those chambers. I will also cut my chambers with HS that is .001 - .002 under my target dimension - this accounts for lug set back in new extensions & bolts. If I rebarrel a bolt gun with the same rcvr/bolt - I cut my HS to exactly what I want - the rcvr/bolt have already been conditioned. I go under .002 if using a new rcvr/bolt.

My headspace gauges are some of my most valuable tools - I wouldn’t even attempt to chamber without them. I keep full sets in 223/556, 308/762 (.001 increments) and Go/No Go gauges for 20+ other cartridges.
I don’t go near a reamer without them.

My .02

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Thank you very much for sharing. Definitely great info. It always surprised me at how far below SAAMI minimum factory ammo measures with headspace gauges.

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Sorry for taking so long to get a few pics… here they are:

This is at 2,031rnds

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That unfortunately appears to be some poor quality anodizing. Type III Hardcoat - which is what should be used on these parts can by standard vary from a half a thou (.0005) to .0045. Carriers should NOT be a tight fit - there should be a level of “floating”. Bear in mind as well that trigger assemblies can exert some measurable upward thrust to a carrier as well. Most of the the good quality uppers I have worked with and inspected also have a DFL (Dry Film Lubricant) applied to the interior. Anodizing in itself can be a tricky business - hugely dependent on operator QC. Weak baths, current variances and bath temperature are critical. Hardcoat - properly done needs to be a “cold” bath - this requires a really good system to cool it. Standard anodizing will heat a bath significantly. I suspect that this isn’t a true hardcoat Type III anodizing. You can also see the machining marks from when the upper was bored for the carrier.
You may be able to have the interior treated with one of the high adhesion DFL coatings. This may slow the process of the carrier rails really wearing on the almost bare aluminum.

ETA: Please don’t offense at my opinion - I would expect that an upper with that few rounds through it would show far less wear. In reading the whole post over - it appears that you were not critiquing the wear- but trying to show how slick the upper was. I have some uppers that have in excess of 15K rounds through them and they show far less wear than that.

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No, I’m actually really glad you said something. I always wondered because since day one that Upper had me wondering, cause it never even had the Dry Film Lube in it, like my BCMs have all had. That is a Vltor MUR-1a Upper. I had always wondered what that Gold showing through was about it.

When lubed properly, she does feel very smooth and slick. I’ve never had a Malf., knock on wood.

Is this something I should be contacting Vltor/MSTN about?

Thank you for your knowledge and willingness to share it. No offense taken whatsoever.