Odd, but consistent groups that I can’t explain

[LensWork]

I am getting some very odd groups with my new custom Aero Precision M5, in .308 Winchester. With several match grade factory loads at 100 yds., I am getting three rounds of a five round group that are a cloverleaf measuring ~ .25” center-to-center, with the other two rounds high and expanding the overall group to ~ 1”x1”. I have gotten these virtually identical groups with Federal 175 7.62x51 MK HPBT, Nosler 175 Custom Competition, Barnes 168 TTSX and Lapua 175 Scenar-L.

The groups are fired semi-auto, magazine fed. The groups have all been fired at about the same pace: one round every 30-60 seconds, so no significant heat-cycling of the barrel. I am getting the same results from multiple different magazines  — all Mag-Pul — 10 round, 10 round with -5 round limiter and 25 round M118LR magazines, so I can’t attribute it to a specific magazine. The barrel is a Criterion 18” Hybrid; comp is a Precision Armament M4-72; BCG is JP Low Mass with a JP Enhanced bolt; optic is a Trijicon Credo HX 2.5-15x56mm. I have duplicated the same results on different trips to the range. 
 

I am at a loss as to explain how I could be getting 3/5 rounds in essentially one ragged hole and the other two high “flyers”, with multiple different factory loads. Any ideas?

[jtallen83]

I had a gas block that was just barely touching the rail when fired, that gave me some erratic groups until I found it.

[shooterrex]

Scope mount loose? Barrel nut not tight? Gas block touching rail?

[Armed Eye Doc]

Is it the difference between left and right side of the magazine?  Try one round in a mag for each of five shots so they all come from the same side.  

[LensWork]

3 hours ago, Armed Eye Doc said:

Is it the difference between left and right side of the magazine?  Try one round in a mag for each of five shots so they all come from the same side.  

That is what I thought it could be; I am getting the same results using multiple different magazines, so if it is related to the round being chambered from one side of the magazine, and it’s not the magazine itself, I wonder if it could be the barrel feedramp???

[jtallen83]

15 minutes ago, LensWork said:

I wonder if it could be the barrel feedramp???

Like in tearing up the bullet, that would be easy to figure out anyway.

[98Z5V]

1 hour ago, LensWork said:

...so if it is related to the round being chambered from one side of the magazine

That will immediately indicate an issue with the ejector face...

[98Z5V]

P-Mags.  First round is left, top round is right - always - like it should be in a mag. 20 rd mag, last round is right.  30 rd mag, last round is right.  Been that way for decades, before Big Army fucked that up.  Ignore Big Army.

You loaded 5 rounds, in a P-Mag.  First was left, etc.  5th was left.  No other way around that.  It is what is is.

You had 3 rds clovered. You had 2 rds out, but close (but you haven't posted any pics or your groups yet...  just of your pretty gun...)...  <<<  BIG deal right there. Get your group pics up.  Post 'em.

You have a problem with the profile on your ejector face.  Nothing more. 

Post pics of the face of your ejector, so we can look at it. 

[98Z5V]

@LensWork, let's get back to the beginning, on your build... 

 

[98Z5V]

@LensWork, you were on here tonight - did you read this thread again, whilst you were on the board? 

[LensWork]

Working on the theory that the issue was related to the feeding of the cartridges, I went back to the range last week and asked my wife to plot the impact locations using a cheap (but for this purpose effective) spotting scope. It was immediately apparent that the “fliers” were inconsistent in each shot string, eliminating the hypothesis that the issue was related to the position(s) of the rounds in the magazine, or a defect on one side of the feedramp.

The cause of the fliers was still not apparent until I shot to qualify to use the longer — greater than 100 yards — ranges at my nearby FWC range. In Florida there are a number of ranges scattered around the state operated by the Florida Wildlife Commission (FWC). To shoot on the 200 yard, and longer ranges, you must qualify by shooting five out of five rounds within a 7” target at 100 yards — I know piece of cake 😉. During qualification the only support allowed is a bipod (which although pictured in the initial post, I was not using during the previous range sessions), no other support is allowed.

During all my previous range sessions with my new .308 AR, I had been using a Caldwell® AR DeadShot Tactical Bag Set, which consists of a bag for under the handguard and a rear bag that is designed to fit either under an adjustable stock, or flipped over, fits under a fixed rifle stock. Due to recoil, the rear bag would usually need to be repositioned after each round was fired. Also due to the design of the rear bag, and the adjusted length of the MagPul SL-S stock for proper eye relief with my scope, a firm and consistent position of the buttstock within my shoulder pocket and a repeatable cheek weld was difficult to achieve.

I believe that this is what lead to the fliers; I say this because during qualification, I shot the best 5-round group that I ever have from my new rifle: 0.79 MOA with Lapua 175gr. Scenar-L factory ammunition. Without relying on the bags, I was able to achieve a firm, consistent shoulder-pocket location and pressure, gripping the front of the magwell, with my thumb wrapped over the top of the handguard, and pulling the rifle into my shoulder, and also a repeatable cheek weld.

As a former Marine — qualifying expert with the M-16A1 every year of service — this should have been a no-brainer in diagnosing the cause of the inconsistent grouping, but forty years hence, and a 25 year “vacation” from target shooting, it took longer than it should have to solve the problem.

I do thank everyone that offered input and suggestions on the cause of the problem, and how to possibly rectify the issue. Also, though I was aware that recoil starts the instant the bullet begins its travel down the barrel, this experience reinforced that knowledge. The bag set that I was using was great for consistent reticle/target alignment, but since my rifle was basically just laying on the bags, and that during most shots I was not applying any pressure to hold my rifle in-place as I pressed the trigger, the movement under recoil, as slight as it might have been (my M5E1 weighs ~11# as pictured), made a definite, measurable difference in point of impact.

[shooterrex]

Glad you got it figured out.

[LensWork]

1 hour ago, shooterrex said:

Glad you got it figured out.

So am I 😉 I just wish I figured it out $400 ago! (~ cost of ammo, range time and gasoline back and forth to the range expended during testing). I just didn’t think that not firmly holding the rifle while pressing the trigger would have that much effect on the POI. I thought that 11# resting on the bags would yield consistent results. Expensive lesson (re)learned.

[shooterrex]

Marksmanship is all about doing the same thing the same way EVERY time.

[98Z5V]

16 hours ago, shooterrex said:

Marksmanship is all about doing the same thing the same way EVERY time.

Correct.  Fundamentals. 

[98Z5V]

We were trying to diagnose a rifle problem, over the internet, based on a description of what was happening, by the shooter.

What we ultimately had was a problem in the Fundamentals of Rifle Marksmanship, purely from the beginning, by the shooter.  Entirely. 

I recommend this for a required reading list.  And not just READING...  UNDERSTAND IT, absorb it, and PRACTICE IT...  then you can try your hand at Basic Rifle Markmanship.

If you can find an old copy of the real Army FM 22-9, that'll do you good.  That's the last one before it got crazy, and started new FM numbers.  In the meantime, this one will do:

https://www.amazon.com/Manual-3-22-9-Marksmanship-Weapons-February/dp/1475198299

Basic Shooting Fundamentals are FULLY covered in this manual.  START with basic shooting fundamentals, before you request that we diagnose "gun problems" over the internet - make sure you can SHOOT, first.  Then we'll talk... 

Edited July 17, 2022 by 98Z5V

[mrmackc]

On 7/17/2022 at 12:18 AM, 98Z5V said:

We were trying to diagnose a rifle problem, over the internet, based on a description of what was happening, by the shooter.

What we ultimately had was a problem in the Fundamentals of Rifle Marksmanship, purely from the beginning, by the shooter.  Entirely. 

I recommend this for a required reading list.  And not just READING...  UNDERSTAND IT, absorb it, and PRACTICE IT...  then you can try your hand at Basic Rifle Markmanship.

If you can find an old copy of the real Army FM 22-9, that'll do you good.  That's the last one before it got crazy, and started new FM numbers.  In the meantime, this one will do:

https://www.amazon.com/Manual-3-22-9-Marksmanship-Weapons-February/dp/1475198299

Basic Shooting Fundamentals are FULLY covered in this manual.  START with basic shooting fundamentals, before you request that we diagnose "gun problems" over the internet - make sure you can SHOOT, first.  Then we'll talk... 

Downloaded this onto my Kindle account to all my devices, Desktop, apple smartphone and 3 or four Kindle Fire HD Tablets for $2.99  Thanks for the LINK!

[Cunuckgaucho]

 

Let's see if this works

https://www.benning.army.mil/infantry/DoctrineSupplement/ATP3-21.8/PDFs/fm3_22x9.pdf

 It's 22mb 

Edited July 19, 2022 by Cunuckgaucho

[LensWork]

https://forum.308ar.com/profile/16921-cunuckgaucho/

https://forum.308ar.com/profile/15401-mrmackc/ 
 

Thank you very much for the link(s) 😉

 

I had never shot from a bench prior to building and test firing my M5E1. I had also never used any sort of support, like a bipod or bags. Additionally, I had never shot using a magnified optic before. All of this combined with a nearly 40 year lapse since my Marine Corps days (M16A1, Commando), I think that I relied too much on technology and forgot basic marksmanship.

When I first experienced the “fliers”, my initial thought was that the weapon’s recoil was to blame; that the muzzle was rising while the projectile was still traveling down the barrel. I discounted this hypothesis, partly because of the consistency of the groups (3 of 5 shots into a sub 1 MOA cloverleaf, and two high shots), and also because of my inexperience with this type of equipment and style of shooting. I just found it hard to fathom that the combination of supports, total weight of the carbine and  — by all reports — a highly effective muzzle device (Precision Armament M4-72), that the weapon would move enough in the ~0.0005 seconds of the bullet’s travel down the barrel to affect POI that much.

Boy was I wrong! Once the bag supports were removed for the FWC qualifying, and I was forced to recall, and revert back to the basic principles of good marksmanship, my group size shrank and the “fliers” disappeared. The five rounds that I shot for qualification were the last group that I fired that day, and as of this time, the last group since. I am anxious to get back out to the range, and with the lessons learned, see just what my new  .308 AR is truly capable of.

[LensWork]

I headed back to the range on Sunday, and using the lesson learned from my qualification set-up, I shot a group that I am quite proud of. I can honestly say that I am not really surprised given all of my previous groups that had three of five rounds in a cloverleaf that measured 0.23” - 0.28” center-to-center. By eliminating the fliers (thanks to those that suggested that I was not being consistent in my set-up), I think that I have wrung the best possible accuracy out of my new rifle. I would have been happy with a 1 MOA gun; so I am thrilled with 0.5 MOA.

There is a significant shift in POI between shot 1 — with a cold, clean bore — and subsequent rounds, but the POI remains consistent from shot 2 to shot 20 and beyond.

[FOGeologist]

On 6/21/2022 at 4:57 AM, jtallen83 said:

I had a gas block that was just barely touching the rail when fired, that gave me some erratic groups until I found it.

You know something? 

I had read your response before, but never considered this could be happening to me.

Then, I grabbed the skinny 18" Faxon Big Gunner barrel on my rifle and flexed it... and the lop-sided, weirdly-shaped PSA adjustable gas block hit the inside of the handguard on the left. 

I know I'll be replacing the Guntec handguard soon, so I decided to hack off the top 3" of it. Now no amount of flexing of the barrel will allow it to hit the inside of the handguard. 

We'll see if there is any beneficial effect come next shoot.

Edited October 17, 2022 by FOGeologist