Weird grouping issue

[dellet]

Load a full magazine,
Shoot three,leave a round in the chamber, wait how ever long
Shoot three, repeat.
This will give a better idea if it's temperature or cycling speed.

Chamber a round as you normally do, then pull it.
Shoot two or three and allow another one to chamber.
Pull that one and compare length to the first one you pulled.
This will possibly tell you if length is changing when feeding,

Check both pulled rounds for concentricity, you might be able to see that by just rolling them on the table. Look for a wobble at the tip.

I would also try shooting a group by loading loading a full magazine, shoot one round.
hand cycle the second round out and shoot the third, remove fourth, shoot the fifth.

What buffer are you using?

If you have a heavier one, I would try it.

I would lean toward something happening during feeding because it's cycling too fast.

[Grouse870]

It’s free floated. I’ll try that. Just a standard carbine buffer. But if it’s cycling too fast wouldn’t that show up in the second round not the first

[dellet]

The first round cycles the slowest.

If you have a pistol gas system and using a standard carbine buffer, 2.7 OZ, it's definitely cycling too fast under fire. I would got to at least an H2, but probably an H3.

Basically you're over gassed and under buffered.

Some of the other tests would likely show that by shooting all hand cycled together or all gas cycled.

[Grouse870]

Hmm. Didn’t think of that. That actually helps. So H3 would be your recommendation?

[dellet]

Hmm. Didn’t think of that. That actually helps. So H3 would be your recommendation?

The system was designed to use an H2 with pistol gas. But that allows for it to operate with subsonic/low pressure ammo. Basically a bit slow for subs and a bit fast for supers, best compromise.

Personally I would lean toward the heavier H3, but you will likely see a difference with the H2. If possible I would beg, borrow or steal from another AR or a friend if possible just to try.

The other thing to consider in a buffer is a shot or powder filled one. Solid ones will bounce off the tube. Powder or shot filled are like a dead blow, They stop and then the spring pushes them forward. Spikes makes them and they are the same weight but it would be a "T"2/3 instead of an "H".

[rebel]

More than likely, dellet has it pegged. Much greater mass here, but I saw a friends NEMO in 300 WM move a bullet out .030 just by chambering. Enough to do exactly what you are seeing. All his other shots wadded right in the group. Neck tension is a balancing act in a semi, need enough to hold the bullet still, not so much it shoots like crap. Softening the severity of the blow sure helps with that.

[Bob the nailer]

After you change buffer tube springs if the issue is still there I have had good accuracy results by tuning the barrel nut torque. This is a big debate on effective or not however all I can say is several AR's I've tuned have improved with consistency of shot placement.

[Whole Bunches]

Interesting thread, as I am having the same problem.

New build with Bear Creek side charge upper and 16” Green Mountain Blackout bbl. Using my Barnes 110 gr blacktip loads, 2.25” oal, the first round hits 1” low at 34 yd, 1.5” low at 50 yd, and 3” low at 100 yd. The rest of the group is higher and groups tightly. Many groups fired hoping it would get broken in and stop throwing the first round loaded low...200 rds later, first shot still low.

Does not matter:

How many rounds are loaded in the mag

Whether the first round is hand loaded into the chamber without using the mag

Whether the first round comes from multiple mags

Whether the first round is loaded by pulling the bolt back and letting fly or whether the first round is loaded by pressing the mag catch on a locked back BCG

Two lowers tried

Two buffer springs tried

Standard carbine and H2 buffers tried

Upper taken apart and put back together

Bbl clean or dirty

Bbl hot or cool for each shot...bbl and suppressor cooled by fan to ambient temp between shots for the bbl cool testing

Good scope mounted tightly

Different powder charges tried

Two bolts tried

Frustrated after two range trips, I rapidly fired 5 of my plinking load using pulled 7.62x39 bullets and the first shot was not low! While the Barnes is my first choice and go-to hunting bullet, I decided to try some Nosler 125 gr Ballistic Tips. Nice groups with the first shot not low. Go back to Barnes...first shot low. Go back to Nosler...first shot not low. Multiple groups with both bullets gave those consistent results. Even did one group with nothing but “first round loaded” Barnes...nice group, just low.

I used the Nosler for years until the Barnes became readily available. Barnes give about 2” more penetration on boar hogs over about 150 pounds on body hits. On deer, both bullets expand and go right through. On sows, both bullets expand and go right through. Barnes expand better on coyotes.

I did not try ejecting the first round to examine it, but this thread tells me I need to do that. I do give the Barnes bullets a mild crimp. Feed ramps on upper and bbl extension look fine to me. Barnes loads are 2.25” oal. If Nosler loads are set to 2.1” oal, about 20% do not feed. At 2.2” oal no jams with the Nosler. I keep both oal’s of Nosler loads on hand as other Blackouts will have a preference of oal for feeding or accuracy or both.

I do not think I have a heavier buffer than the H2. I forget the gas tube length...I am not home right now to check, but think it is carbine length. I shoot it suppressed with supersonic loads.

PS I am the guy measuring Blackout bbls for throat wear and shoot thousands of rounds. The GM bbl was new and gaged new and came direct from GM. I might try putting the upper on a rifle buffered lower. I will also see what carbine buffers I have on hand. I think I have std, H2, and Spikes T2.

[dellet]

Something to note:

Whole Bunches- reloads, hitting low

Grouse870 - factory, hitting high

High and low may not mean a thing, but factory and reload might suggest rifle more than ammo.

One thing I will throw out there, almost all of my unexplained flyers disappeared using 296 once I got to about 97% load density.

[Whole Bunches]

My Barnes 110 gr load is normally 20.0 H110, but did try 19.9 and 20.1 H110. Nosler load is 19.0 H110. Caution, do not use that Nosler load if the oal is less than 2.2”! Start low and work up for your gun.