OP's 1st post,
An AR will shoot 'One Hole' groups, but the same rules apply as with bolt rifles.
This leans into 'Gunsmithing' a little bit, but stay with me...
AR's inherently don't align the barrel with the upper receiver, this causes the bolt carrier/bolt to NOT line up,
And it 'Cocks' the bolt 'Sideways', keeping the bolt from locking up square with the locking lugs in the chamber nut on the barrel.
Now, the barrel and your optics/sights both attach to the upper receiver,
The barrel is cocked a little sideways to the upper receiver,
And the optics can EASILY wind up being cocked a little sideways on top the upper receiver, creating TWO lines that don't properly connect,
Line of sight, line of bullet travel as seen from above...
There is only one way to eliminate this,
Have the bolt carrier BORE in the upper honed to be 'Align Honed' (Honing, only the 'High' spots removed),
Use that bore to align a square cut on the front of the receiver so the barrel isn't cocked even slightly to bolt carrier/bolt.
The front lip that is 'Supposed' to be square to the bore is usually 0.004" to 0.040" off 'Square'.
0.010" to 0.030" is QUITE common...
IF the barrel/chamber nut CAN NOT get square with the bolt/carrier, the locking lugs are never going to lock up evenly,
And the barrel is NEVER going to align with the upper receiver bore, Or the optics mounts...
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The next thing to consider is the top end and bottom end 'Float' on each other.
Tightening up that slop will keep the upper from moving on the lower, something like an 'Accu-Wedge' or expanding break down pins will stop a bunch of this, keeping the upper from sloping around on the lower as the hammer swings, hits the firing pin, and the firing (internal ballistics) happen.
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With your barrel twist rate being so 'Fast' or 'Tight', you should start showing increased accuracy around 55-60 grain bullets.
A little longer bullet has more 'Load Bearing' surface on the sides, so it holds the rifling a little better than short bullets do.
The bolt rifles usually have 1:10" or 1:12" twist (even 1:14"), shooting lighter bullets more accurately.
Short, Light 'Boat Tails' remove bearing surface from the rifling contact.
Short, light, boat tails pushed fast through a fast twist rate barrel will strip 'Strings' of jacket material off the bullet, so the bullet is NOT balanced and doesn't have the proper 'Spin' coming out of the barrel.
Flat base bullets light weight bullets in fast twist barrels *Usually* shoot better than light weight 'Boat Tails'.
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The next thing is your mags,
Suck up all the 20, 10, even 5 round mags you find.
They feed better,
You usually aren't going to bench shoot more than 10 rounds at a time,
And they are CHEAP since the mag capacity ban expired,
But they are getting hard to find in a lot of places, particularly the really short ones with the plastic bottoms...
30 round mags on the bench are a super pain in the butt...
Optics have to mount on a tall receiver anyway, so off the bench you wind up with a super tall bag stack to start with,
Then trying to clear a 30 round mag makes it even more akward and unstable.