Odd groupings...whats happening?

T

Taco-XL

New member
I shot my first handloads today :D. Was using my AR 308 i built. 168gSMK, IMR 4895. Started at the min of 38 and worked in .5g increments up to 41.5g, 5 shots each. I got phenomenal results right out of the gate with the 38g, under 3/4 inch at 100yrds.

This is a two part question. A) i noticed the lower charges were hitting low, which i expected, but they were also grouping to the right. As i went up in charge, the groupings moved back closer to center. Whats this mean? and B) i also had almost-as-good groupings at 41.5g as i did at 38g, but everything in the middle was just, meh. I found it odd to have two good groupings so far apart and the middle stuff not as good. Is this normal to have two 'sweet spots'?
 
It's normal to have a few sweet spots testing ammo at 100 yards. There's a 3-grain spread in 4895 charge weights under 168's that are virtually equal in accuracy at short ranges. It's easily seen if you shoot 20-shot groups with each load a grain apart. I used 39, 40 and 41 grains under 168's in matches at 200 and 300 yards that when properly tested shot under 1/2 MOA at worst.

It's also normal to have different group positions on paper testing different loads in each one. We humans are not too repeatable shouldering rifles exactly the same for each series of shots shooting from a bench.
 
I would als mention heat.
As the barrel warms up, it will shift zero from cold bore shots.
Depending on the work you have done to it, you can minimize the cold bore shift, but it will never go away entirely,

The amount of time the rounds heat soak in a hot chamber will cause the ammo to shoot differently also.
 
None of the 1971 military team members shooting match conditioned M16's in the first match they were allowed in mentioned zeros changing as barrels heated up. Mine didn't; I was one of them. Nor did our 7.62 NATO M14's nor Garands.
 
I let my barrel cool before each new load, and i didnt load the new rounds into the mag or chamber until ready to fire
 
That's sure not what every DOD study found that I've seen, and there have been HUNDREDS.

I fixture mounted rifles, removed the stock, clamped the recoil tube and test fired a bunch of rifles with shifting zero complaints,
And virtually all AR style rifles shift zero when they heat.
(Edit: recoil tube and receivers clamped, barrel left free to move)

How well upper receiver, barrel, barrel nut line up,
And how much hand guard load is on the barrel aggravate the condition,
Or reduce it...

'Service' rifles aren't SERVICE RIFLES anymore in competition,
Cherry picked receivers, full float barrels, anything anyone can get away with is allowed...
No point in calling them 'Service' rifles when EVERYTHING is modified/tuned, and usually custom made.

Custom chamberings in custom barrels, non standard rifling, aftermarket triggers, full floating barrels,

Anyone that says heat doesn't move metal around, is having issues with phsyics.
The reason we spend so much time truing/squaring in the first place, to direct heat expansion/contraction forward instead of sideways.

Not only does heat make metal expand & contract, but it changes the harmonics/vibration patterns...
Something you guys dove into on another thread but never mentioned.
 
Now, granted, we didn't get the rifle unless there was a complaint.
Most don't shift dramatically, a few do.
When someone complained enough, we got it, and with Marines being the riflemen they are, they were normally right.

Defects in the grain structure, poor machining, and out of square uppers were usually to blame.

'Rapid Fire' in civilian matches and actual Rapid Fire are two different things.
Since a BUNCH of the barrel heat winds up in the chamber, migrating to the upper receiver, that upper to barrel fittment is critical if you want to shoot in a stright line, that's why I stress it...
 
Taco, I love the AR 10 platform,
The same rules apply with it as any rifle, you can tune it up.

It always makes the turn bolt guys snicker when I break out my AR 10,
But after I run right with them out to 600 yards, the snickering stops,
Then at 800 they start cussing,
Now, we both know that 800 is stretching it,
But with some serious tuning, I reach for 1,000 and the turn bolt guys pack up and go home!

It's not a dead eye 1,000 yard rifle, no AR 308 at the local makeshift 1,000 yard range is...
But I can get on the targets, usually in the black,
Where the 'Thanks For Comming' off the shelf turn bolt guys can't get on the target at all...

Just to add insult to ego injury, I'll uncase my Knight Armorment M 110 now in .300 Win Short Mag just to see the looks on the faces, and see how many more pack up!

Us AR 10 guys are often the butt of jokes, so it tickles me to no end to hear you are putting lead on target!
 
My answer will be different than Bart's.

Smaller charge could be the drop you saw, since the rounds came up progressively as charge increases.

Assuming enough bolt open cooling time between rounds,
Consider this,

When you fire that rifle, the end of the barrel makes an egg shaped vibration 'Circle' as the vibration from ignition makes a shock wave in the steel of the barrel,
As the bullet is forced through the barrel, ROTATING in the rifling,
The barrel, already in motion from the chamber pressure wave, the bullet hitting the throat of the rifling,
Starts to do an 'Oval' circle at the muzzle.

Each progressively faster round found its way out of the muzzle at a different point in the 'Oval' movement pattern...

Since you didn't have big jumps in charge/velocity,
Vibration would have stayed about the same, while the bullet reached the muzzle at a slightly different point on the 'Oval' vibration pattern,
Leading to a consistent movement in point of impact.

Laying a wet towel on the barrel will do two things,
Dampen vibration/cycling of the barrel,
And keep the barrel a more consistant temp.
Our service troops do this as part of 'Field Craft' and they shoot farther while staying consistant when they get barrels that 'Walk' point of impact with temp or ammo changes, like ammo heating up during a desert day.

There are also those goofy looking rubber vibration dampers on the market.
If it worked, I wouldn't care how goofy it looked!

In the old days of bench shooting, guys would actually cut a 1/4" off the barrel when they had vibration/cycling issues.
Now they use stupid heavy barrels to control cycling...
 
Mechanical engineers working with shake and vibration analysis see no difference in a rifle barrel's whip and wiggle characteristics going from cold to hot then back to cold. Barrel temperature's not an input to their calculation software when seeing what will happen.

Many people have observed no change in bullet impact at 600 yards with the barrel temperature going from ambient to too hot to comfortably touch. 40 shots inside 2 inches at random points 600 yards down range seems to say cold to hot barrels don't change their vibration characteristics and shots cluster about the same place.

JeepHammer, one request: your posts would be easier to read if you didn't press the [Enter] key after each press of the [,] key. Same thing regarding the [.] key.
 
So now you are upset with the way I type, Bart?...

Comma [,] is a slight change in information, or thought in a sentence.

Period [.]ends the sentence or thought.

New thought in new paragraph, although I don't indent paragraph starts, I probably should...

High speed video footage CLEARLY SHOWS the muzzle oscillation, usually in a lopsided oval, more of an egg shape.

-------

os·cil·la·tion/ˌäsəˈlāSH(ə)n/
noun

1. movement back and forth at a regular speed.

2. regular variation in magnitude or position around a central point.

-----

When high enough speed cameras came out,
Suddenly barrels started to get super stupid thick...
Which stands to reason, since thicker barrels don't oscillate as violently as thinner barrels.

Now, if you don't want a 2.5" or 3", 30 pound barrel blank to make your .22 cal rifle out of,
And I can't imagine anyone but a bench shooter would,

I was one of those guys, 2.5" barrel, no contour cut. Could see the bullet strike the target, but you don't want to lug it around!

Then you try and make the barrel as structurally as rigid as you can,
That takes paying attention to grain structure and materials that don't resonate as much as some others in the past have.

Again, anyone that thinks a barrel doesn't 'Ring' when you stuff a hunk of metal down it under tremendous pressure, while trying to spin that hunk of metal at the same time.
That hunk of metal grinding on every imperfection in the barrel, the edges of the rifling, fouling in the barrel, ect.,
That guy isn't paying attention to basic physics.

EVERY friction point will cause an oscillating front in the steel,
Not to mention the swelling/compression of the barrel as the pressure and bullet move through a 0.006" SMALLER bore than the bullet diameter (in a .223 rifle, 0.218 Rifling Land bore, .0.224 diameter bullet)
The act of the rifling compressing the sides of the bullet INWARD will cause a swell in the barrel rifling/barrel as the bullet passes through.

Again, an oscillating vibration running through the barrel, along with the oscillations created from the metal grain expanding/contracting as the bullet passes.

So I'm not sure I'd invest too much in an 'Article' someone published that says 'Their' barrel doesn't oscillate...
Since all barrels oscillate to the point of seeing the oscillations on high speed cameras.

Tank barrels use computers to track oscillation in real time, compensate for that oscillation in the next firing formula.
We're not exactly tank caliber here, and not shooting 5,000 yards, but the principal is the same, we just don't track it...

AND!
If you would have read what I wrote,
You would have seen I said we would disagree.
I didn't directly attack what you wrote, But you were ready to crap on anything anyone else might have to say...

I SUGGESTED the guy try a wet towel to dampen the barrel for a few test firings...
Nothing lost, easy to do.
If it helps, then it's something that works,
If it doesn't, nothing lost... He was going shooting anyway.

Funny, All these things that field troops, Marine Corps have found useful, tried and true all these years,
And you crap on every one of them since you didn't do it in your 'Match' shooting, or didn't read about it on some internet forum until now...

Sometimes barrel dampening works for me,
Sometimes it doesn't.
If it works, I might even be temped to cut a little off the barrel and re-crown just to see if it changes things...
You know, the way guys have been doing it for the last couple of centuries...
Worked then, still works now when you have 'Issues' that can't be cleared up any other way.
 
Bart B. said:
None of the 1971 military team members shooting match conditioned M16's in the first match they were allowed in mentioned zeros changing as barrels heated up. Mine didn't; I was one of them. Nor did our 7.62 NATO M14's nor Garands.
Click to expand...

You mean your National Match rifles?
All squared up/true receivers?
All hand fitted bolt to receiver rifles?
All carefully head spaced and chambered?
All hand fitted gas pistons/operating rods?
All gaged and hand lapped barrels, everything tuned to be a precise shooter?
All glass bedded or resin impregnated stocks?
All hand built by qualified, master gun smiths?
And you DIDN'T have problems with a hand built rifle?

I'm SHOCKED! :eek:
You could at least hope for a paper cut taking the wrapping off when they got to you from the gun smith!...
Get roller grease on your fingers or something...
 
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I do not know about any one else but I am eating this up. Very good coverage of a complicated subject in a very understandable discussion. Thank you.
 
Kind of a common physics thing.
Physics, when things MOVE.
When something moves, it interacts with everything that comes into contact with it.
Force/Motion/Friction/Action/Reaction.

Then there are the common sense things,
Pampered, Stroked Competition rifles built by master gun smiths are going to work better than production line, ground out and assembled by fairly unskilled production line workers.

Taco said he BUILT HIS OWN RIFLE, (BIG +1 From Me on that one!)
And he built his own rounds, (another +1)

And the rifle/rounds did very well, he did several things 'Correctly' or he wouldn't be seeing the sensitivity he is in the load differences.
Sloppy rifles don't show slight changes in load, they scatter bullets no matter what the load is...

'TACO' DID NOT SAY IT WAS A PAMPERED, STROKE BUILT BY A MASTER GUN SMITH, which most national match rifles are.
Shooters can't seem to understand what it takes to put the rifles together since virtually no competitors built their own rifles anymore, with maybe the exception of bench shooters...

The more you build, the more you learn, and I'm glad to see 'Taco' in the AR 10 fold!
He's off to a good start, and should just get better from there!
 
Indeed, i am no gunsmith nor claim to be. But i am fairly knowledgable on the AR platform and have a decent concept on how to build them correctly and not cut corners. Its not a competition rifle that i had to sell a kidney to build, but i do feel i put enough attention into the components to be a respectable bench gun.

I dont feel like heat was my issue. I waited til the barrel/gun was cool to the touch before i shot the next load. I cannot put wet towels on my barrel as i am running a rather lengthy hand guard. I have never done any load testing, as these are my first rounds down range handloading. I just wondered if my experience was the norm, i just assumed (maybe wrongly so) that the accuracy curve would be more gradual and definitive. I will however take my 3 most accurate loads i found and give each more rounds to get a better idea if this was a fluke or something concrete. I also have IMR 4064 and 175 grainers to test as well, so i still have a good bit of data in front of me to line this baby out.
 
Velocity changes can explain the rise vertically as load increases.
Not a monster shift, but a steady, gradual, fairly even rise as time from muzzle to target lessens,
Gravity has less time to work on the bullet in flight.

Because you explained to us in a rational manner what happened with GROUPS,
Not just rounds,
And we can figure out the vertical movement as velocity change,
The windage change needs to be explained...

Windage plus elevation presenting as diagonal movement.

Since you let it cool, and it grouped with each respective load,
I would be inclined to believe you let it cool enough.
Heat expansion stringing usually doesn't group, the rifle will string the group out.
When you have a heat torque problem you usually shoot horizontal line, diagonal line or vertical string no matter what load you are using...

I would suppose a gouge in the muzzle crown could cause a gas jet to cock the bullet in flight, getting worse as pressure increased,
But that's not how bad crowns present most times.
Normally bad crowns just scatter bullets.

And since you built your rifle, a bad crown would have probably been noticed.
It's the guys with the flash hider already installed that don't notice a hacked up crown.

Are you using a flash hider or muzzle break, sound suppresser can, anything like that?
On my field rifles, I like a Smith Vortex to break up muzzle flash in low light conditions, and the muzzle crown protection doesn't hurt anything either!
With a Smith you will sometimes need to use shims between barrel and flash hider,
The open 'Finger' prongs on the Smith will give me a barrel harmonics change.
And shims will negate or eliminate any shift I see at the target.

Closed end brakes/flash hiders (no 'fingers') usually don't mess with groupings too often...

Other that barrel harmonics or crown angled or damaged its hard to explain the side shift movement you are seeing.
To group, the movement is controlled and precise, repeatable,
So that means its something the rifle/ammo is doing over and over again.
The only change was slight powder increase, and the way that barrel responds to slightly increased pressure. Again, repeatable results on a GRADIENT as you increased pressure...

I would take this on as a barrel harmonics issue rather than ammo or some outside viarable.
The trick, and absolute proof its a barrel issue,
Can you do it both ways?
Start with lower charges, increase the charges and have the impact shift,
Then Reduce the charges in reverse order and repeat the shift on demand...

Then, try a different but similar velocity powder charge,
See if it does it again....
This would dead nut bolt it down as harmonics if you could reproduce the same effect in two different powders.
 
All for fun here, But Jeephammer- I am a turnbolt guy that would LOVE to take you out to 1000 to 1400 yards. We can see who would be packing up first:D
The AR's have come a long ways in the last 10 years, but lets not get overboard here. Still are and probably always will be a short to medium range set up. I see a few more every year in the matches, but still not on the same playing field as the turnbolt guys.
 
As for a wet towel, the field guys usually put it pretty far out towards the muzzle,
Sometimes all the way up behind the flash hider.

The idea is to dampen muzzle oscillations, not the root of the barrel.
It's just a quick way for them to tell if its the pronged flash hider, muzzle oscillations, or something else messing with their shots.
When the towel doesn't work, they usually came back and kicked our butts over the ammo... Or rifles if they were new.

Since they were REALLY good marksmen, they were usually right and knew when something wasn't right, not just a hangover day on the range...
 
4 Runner, you are absolutely right!
That's why I mentioned the local make shift long range,
And the local guys with off the shelf production rifles from Wally World or K Mart out chest beating...

Nothing quite like an off the shelf rifle, a 'Bubble Pack' optic and rings set out trying to reach 1,000 yards with 'White Box' ammo...
And my point entirely.

They have never shot 500 yards before, shoot 8" groups at 200 yards at the local public range,
Then see some TV show about long range shooting,
Grab a rifle, white box ammo and a bubble pack optic and head out to the local strip mine or farm field where someone has set up a longer range,
And start blasting away confident they can teach themselves to shoot 1,000 yards in one afternoon, all the while sneaking beers back at the truck...

The only ARs they have ever seen are the $400-$500 farm store 'Tactical' carbines, and seen them spray bullets everywhere,
So when an AR comes out, they snicker, and I make a few bucks on side bets...

Just because they can afford something chambered in .300 mag doesn't mean they can shoot it,
Or the Wally World discount version of this or that rifle is capable of accurately shooting at 1,000 yards...

I can hit a 4'x4' target/back board at 1,000 yards,
On good days I can keep most rounds in the black,
But no AR10 in .308 is a 10 ring tack driver at that range that I can build,
Or have seen anyone build, but I have been out of competition since '94.

My very stroked 300 WSM is set up for 1,000 yards, and not much else,
It will keep most rounds in the 10 ring or pretty close...
It's not exactly your run of the mill AR10.
 
JeepHammer, barrels don't change their resonant frequency nor harmonics thereof in the temperature range they're shot at. If they did, then shooting a .308 Win round every 20 or so seconds 40 times would not put all bullets inside 2 inches at 600 yards without a sight change. Nor shooting a .308 Win that rate at 800 yards and all shots landing under 4 inches. Nor shooting a 30 caliber magnum 30 times at the same rate putting all shots inside 6 inches at 1000. That's been done. Befriend an engineer in the mechanical vibration analysis business then ask him to explain the details.

Regarding JeepHammer’s queries on the 1971 Nationals’ service rifles used by the USN team.....

You mean your National Match rifles?
No, not the USN ones. The Navy never had such labels on their 7.62 NATO Garands or 5.56 NATO mouse guns. Just different Marks and Mods. The rifles never knew what they were called anyway; it didn’t matter.

All squared up/true receivers?
No; neither M1's nor M16's.

All hand fitted bolt to receiver rifles?
Yes; but for minimum headspace.

All carefully head spaced and chambered?
See above for headspace. M1 Barrels were made and finished chambered by Springfield Armory Arsenal in MA; what ever they used was good enough. The barrels were clocked in to the receiver so the gas piston was well centered below the muzzle. Don’t know about the M16's.

All hand fitted gas pistons/operating rods?
Yes for Garands, don’t know about the mouse guns.

All gaged and hand lapped barrels, everything tuned to be a precise shooter?
Standard, single-pass broach rifled 1:12 twist barrels for Garands with gas port opened up locally. The USN Small Arms Match Conditioning Unit (SAMCU) in San Diego air gauged them, those with .3077" to .3079" grooves were set aside for Navy Team rifles and the rest used in recruit training Garands. None were lapped nor broken in; just shot as is. John Clerke made the button-rifled M16 barrels with a 1:12 twist. They were all air gauged for uniformity.

All glass bedded or resin impregnated stocks?
Garands, yes. M16's were just screwed together but had Redfield International rear sights attached.

All hand built by qualified, master gun smiths?
No. I think “Master Gunsmith” is a marketing label and has no relationship to one’s ability to do anything with firearms. Retired USN Chief Petty Officers working at the Unit designed the details then rebuilt them; there was no other label they had except “Chief.” They all felt the same about “Master Gunsmiths.” They spent a lot of time at the Nationals fixing civilian’s Garands that had been buggered up by so called ‘smiths. One said so called ‘smiths are most often “Master Gun Breakers.”

And you DIDN'T have problems with a hand built rifle?
No. Neither did any other Team member winning matches and shooting scores as good as others shooting service rifles. Well, maybe one was an exception. All the M16's post front sights were too wide, so I and another USN team member filed their widths down to appear as wide as the 600 yard target bullseye; 6 MOA. Ther Garands all shot under 1 MOA at 300 yards from accuracy cradles, about 1.5 MOA through 600 yards with good lots of M118 Match ammo, With commercial match ammo or good handloads, ½ MOA or better at 300 and 2/3 MOA at 600. The M16's shot a bit over 1 MOA at 600 with their handloads as we tested them from prone atop bags shooting Sierra 52gr. HPMK’s over IMR4895 in new Remington primed cases; as good as the Army and USMC ones did as they mentioned to me.

You could at least hope for a paper cut taking the wrapping off when they got to you from the gun smith!... Get roller grease on your fingers or something...
Not at all. Not when all’s done right, clean, precise and all other elements of “the Navy way.” Never got one packaged in any way; all handed to me ready to shoot. One I had for a couple years I thought its barrel was about shot out, so I brought it into the SAMCU one Friday afternoon for checking. One dropped in a throat erosion gauge and it read over 4. He went to the barrel rack, picked one with “77" marked on it (meant it air gauged .3077" groove diameter), drilled out the original gas port to .1065", knurled the barrel at the lower band point for a tight, interference fit, screwed the barrel into the receiver he’d already removed and popped the epoxied hand guards off, clocked it in, went through 3 or 4 bolts to get one that just closed on a GO headspace gauge with no noticeable slop. Then fit the gas cylinder back on, found a gas cylinder lock that clocked in snug at 4 o’clock then tightened it and put in the plug. Epoxied the hand guards back in place, checked the op rod for proper fit then bent it slightly to do so, reassembled the rifle, handed it to me and said don’t shoot it until tomorrow after the epoxy’s hardened. All in about 15 minutes of his time. I thanked him mentioning I would shoot it in the next day’s 11th Naval District’s monthly match.

Next day on the firing line, I sighted it at 200 yards from offhand with my two sighter shots, then went on to shoot a really good score winning the match and awarded a Remington Model 11 shotgun.

In closing, about your remark: But no AR10 in .308 is a 10 ring tack driver at that range that I can build... You need to check out what the US Army did in 2012 with a team shooting AR10's in .308 Win. winning and setting records with them at 1000 yards.

http://bulletin.accurateshooter.com...set-1k-records-at-interservice-championships/
 
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