BCM 16" with accuracy extremes

[JodyH]

I'm looking for peoples experience as to why I would have some pretty extreme accuracy differences between different rounds out of the same upper.
I have a BCM 16" with a BCM freefloat Keymod rail, a ACOG TA33 mounted on a BOBRO mount (I've also used a Meopta 1x4 in a LaRue mount).
The lower is a Mega with a Geissele SD3G trigger.

Here's what I'm seeing from 5 shot benched, sandbagged groups:
Federal AR223 55gr. ammunition I'm getting 6" groups (patterns) at 100 yards this same ammo will group around 1.5" from my 12.5" and 14.5" BCM uppers.
Fiocchi 55gr. .223 I'm getting 3/4" at 100 yards basically one nickel sized hole.
PMC Bronze and Geco 55gr. .223 I'm getting 1.5" groups.

What's going on with this upper and Federal AR223 ammo?
It's not a 55gr. thing because it shoots superb groups with Fiocchi 55gr..
It's not a barrel defect because again it shoots amazing groups with Fiocchi and good groups with nearly everything else.
I know the Federal AR223 ammo is 1"-1.5" capable because my other two BCM uppers and my Colt M4 will shoot those groups all day with it.
All my BCM uppers are the same specs (1:7, 5.56, chrome) except for barrel length, they even have the same flash hiders installed.
None of my other BCM uppers are this inconsistent between loads, they'll pretty much shoot every 55gr. .223 or 5.56 into <2" with no problems.
These 6" patterns from this upper with AR223 ammo are confounding me.

Ideas

[WDW]

Well, you know it's not the upper, because it does fine with other ammo. Maybe you had a bad batch of the Federal ammo? I'd let someone else shoot the exact same set up that's giving you problems just to verify what's going on, then I guess just avoid that particular load? My first thought, before I finished reading, was that maybe your barrel but wasn't fully torqued down, but that's not it if it does fine with other loads.

[JodyH]

It's not a bad lot of ammo, I swapped uppers and shot 1.5" with my 12.5" BCM from the same magazine (and lower) that shot 6" out of the 16" upper.
I don't mind avoiding that ammo in that upper, I'm just curious as to what could cause such weirdness.
This is the third range trip I've verified this as well.

[WDW]

I'd definitely have someone else shoot the problematic set up, then maybe shoot it at varying distances, maybe start at 25m & work your way back to see when things start opening up. It may be, that for some reason, that barrel just doesn't like that load.

[Prdator]

Jody,

Man sometimes its just that odd.... I have a Win 70 in 6.5x55, It will shoot any Branes bullet into 8' patterns at 100 but its a 1/2' gun with Accubonds and Matchings..

[1slow]

Jody,
As Spencer said ''Just odd.'' I think sometimes the harmonics of the way 1 load in 1 barrel go together gives weird results. Air guns in particular experimented with trying to tune this.

[rifleshooter]

Have you chrono'd the federal when you get the big groups?

[Wayne Dobbs]

It's not an answer you like but the gun just doesn't like that load and there's likely nothing to do or learn further on that. The fact that it shoots so well with other ammo is a great sign, and an experience I've had with other BCM guns, so just drive on.

[GJM]

I sense that these answers will not make Jody feel especially satisfied. An AR in his configuration isn't like a bench rest or specialized hunting rifle, where we work up a pet load, and only use that load. I want my carbines to shoot everything that has .223 or 5.56 on the box, at least acceptably. If it shoots this load, this bad, it would make me wonder what other loads it shoots the same way. I would prefer a consistent 2 moa AR over the 1 moa/6 moa confused AR any day. I would contact BCM.

[John Hearne]

There is more art/magic to shooting than most of us want to admit. Shotguns are notorious for patterning the same load from the same lot number differently, even shotguns with consecutive serial numbers.

Some times, it just is and you have to accept it.