Go to the BP_MasterAttachment blueprint and create a new variable called SpreadIncreasePerShot and set it to a Float type and tick on instance editable.

Go to the BP_MasterRangeWeapon and create a new variable called SpreadAttachmentPerShotMod and set it to a Float type.

Create another variable called MinSpreadPerShot and set it to a Float type and tick Instance Editable and set its Default Value to 1.

Add the highlighted nodes shown in the image below, location at the top of the image.

The MinSpreadPerShot is the minimum spread per shot a weapon can have even if the weapon has attachments that would reduce it further. Do not set this lower then 0.

You can now adjust the per shot increase spread in attachment blueprint class defaults.

The weapons spread will still be capped by its Max Spread value as well as the Min Spread value for the movement state the character is currently using.