Just dipping my toes into the long range world.

Very often, I end up with something like this, with two clustered groups. This one is just a 5 shot group but it happens to me all the time.

Am I sending it at the top and bottom of heartbeat? Some kind of parallax and cheek weld problem?

Rifle is a Scar 20 on a bipod, prone with SWFA 3-15x42 FFP scope in ADM mount.

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Are you shooting of a bag or a bipod? Are you loading the bipod? Also if your cheek weld changes, that will affect your groups.

Are you able to weight the ammo before you shoot it? (if you don't already reload)

Are you breaking your cheeckweld between strings? Maybe checking a spotting scope, or loading 5 rounds, shooting, than loading another 5 rounds?
If you load a full mag, and just practice your best fundamentals will that happen?

Will it do it with all the same ammo, from the same lot? What distances is this happening at? I know at longer ranges something as simple as a ammo lot difference can shift the POI a few inches, despite being the "same" ammo.

If this is a consistent issue, but the rifle is mechanically sound, then it's most likely as you're starting to guess a habit on your end that's formed.
Fastest way to figure out what's causing it is to isolate as many variables as possible. Same ammo, same lot, scope tight, load a full mag, and shoot every shot without breaking your cheekweld OR checking for where your hitting.  Just perfect fundamentals. If your group doesn't split, do it again on the same target paying close attention to where your cheeckweld was on the first string. If it shifts again you can start dialing in what you're doing.

Is that picture posted sideways?

In other words, is that a lateral or vertical shift?

What distance are you shooting?

I’ll try to answer the questions above.

Shooting prone at 100 yards off a bipod.

Ammo for this particular group was PPU 169 gr HPBT, but this double-group group is pretty common for me with any good ammo. I dabbled I’m reloading years ago but never had time for it and ended up selling all my gear.

All 5 rounds are loaded into a mag and sent one after the other. No getting up and moving around.

I don’t think it’s a hardware problem. I really think it’s something I’m doing. I’m trying to find some precision shooters in my area, but it’s mostly fudds around here.

Also, pic is true orientation.  Not vertical shift.

I'm your original post you said it happens "very often".

Approximately how often?I

Do you ever get a tight 5 shot group?

How about 4-1 splits?

NPA=natural point of aim, and I would start looking here.  If you are pushing or moving anything to line up your reticle on target you do not have NPA.  You should be able to close your eyes while on target, then re-open them, and not have shifted off target.  In fact if you have NPA, your eyes could literally be closed when the shot breaks and it should make no difference in point of impact.  Sounds easy right?  Lol, not really, I still struggle with this.

I would start looking at your position behind the rifle also.  Is your body centerline parallel with the rifle's centerline and square to your target?  Or is it cranked off to one side like old school Olympic High Power shooters.  Line yourself up to the rifle before you even get into position or start loading that bipod.

Trigger manipulation, follow through, breathing, use of bags and a whole list of other fun stuff comes after getting those two things sorted out.  If you can figure these out, you are well over half way there.

FWIW "Fudds" we're shooting small groups before you were born.  Even you were the oldest person on Arfcom.

Is it three in one group, then two in the other, or does it go back and forth between the "groups"?

What distance? How is the range setup?

Sometimes I can get groups like that at 200, usually 300 (not that tight, but the pattern), as we have trees blocking a lot of wind at the firing point which makes wind calling nearly impossible until it gets strong enough to blow trees around. Tiny bit of wind shift that holds constant could do it.

Might it have something to do with how it damages the rounds when feeding? Gas guns are rough on your cartridges. You could eliminate that by single feeding a couple test groups.

A change in how you hold the gun can do it too, shifting your NPA.

I'm thinking it's not barrel heat, or the other rounds wouldn't group so tight. But I wonder if you shoot ten, will they still land 6-4, or go 3-7? Or will they fill in the center?

Are you able to consistently shoot tight groups with other rifles? If so it may just be the gun. Improperly torqued barrel screws are a problem on the SCAR platform. The barrel if not tight could be shifting very slightly and causing those double groups. If you can't shoot good groups with other rifles then the limiting factor is probably you. Make sure stock and cheek placement are the same every time. Trigger press not being straight the the rear may also be a cause. As long as your taking your shot at the bottom of the breath heartbeat will not have that much effect so don't worry about that too much. Check the barrel screws and if not those make sure everything else is tight. The SCAR has a habit of working stuff loose. Good luck