AR load development split group causes?

[Gtscotty]

I recently finished a 6.5 Grendel AR build, and did a quick OCW-ish load development to get a useable hunting load before the end of antelope season.

I found a load that will work for this year, but along the way have run into some split groups where I'll basically have two fairly tight groups of 3+2 that have me scratching my head.

Here are pics of the workup:






I suppose that the vertical split groups could just be from a load between nodes, but it does seem odd to me that they tend to stack into two distinct groups instead of a string like I'm used to seeing.

In the past with bolt guns, I've traced split groups to:

-bedding
-gusty winds
-trigger control problems with light rifles

None of these seem to apply in this case. There was some wind, but not terrible, no bedding on an AR, the trigger is pretty good, and with my can mounted the AR is ~8lbs.

A few causes I've considered:

-CTR stock has some movement and isn't great off of bags, could be causing instability issues (kind of like a bedding problem).

-round, thin handguard torquing on the bags under recoil.

-maybe the split groups are just an illusion from the small sample size.

-bullet/powder combo isn't right for the rifle.

-crappy shooting.


Any input from the resident AR experts? Have you seen split groups with ARs before, any guesses at the cause?

 

[Demi-human]

maybe the split groups are just an illusion from the small sample size.

We are programmed to see patterns, even there aren’t aren’t any.

And yet I have seen “doubling” myself.
I think it’s just small sample size. And a sign of consistency in loading as well.

 

[Varminterror]

I’m with @Demi-human in that whenever I see split groups, I shoot a LOT more rounds into that POA to see if it persists. Rarely will it, and even more rarely would there not be some better option to pursue revealing itself on the page.

 

[Blue68f100]

I've seen it several times when one single feed vs magazine feed. I've had the magazine change the group 4" right and 2" up at 200 yrds. Using a bobsled the groups are 1 moa, POA=POI. Barrel harmonics can do some strange things at times.

Full Float barrel?

Then you hang a 12 oz can on the end?

 

[Gtscotty]

We are programmed to see patterns, even there aren’t aren’t any.

And yet I have seen “doubling” myself.
I think it’s just small sample size. And a sign of consistency in loading as well.

That's probably what it is, this is the only bullet/powder combo I've tried out thus far. I've got AR-Comp and 129gr Interlocks waiting in the wings for after I get an antelope on the ground.

I've seen it several times when one single feed vs magazine feed. I've had the magazine change the group 4" right and 2" up at 200 yrds. Using a bobsled the groups are 1 moa, POA=POI. Barrel harmonics can do some strange things at times.

Full Float barrel?

Then you hang a 12 oz can on the end?

I magazine fed all of the rounds. The can is actually 14oz and not the issue, I run it direct thread on a few other ARs, my hunting rifles and on a muzzle brake on my LR rifle. I've never seen it harm accuracy, and often it helps, especially on light barreled rifles.