I am a huge believer in the ACR retractable/folding stock and GS Designs FAMAE adapter. Granted it works best on the .308 SG 542 with its slightly longer receiver, but so long as you don't ride your hand too high on the Pistol Grip it works fine on the shorter 540 and 543 Receivers. The higher line of the Cheek-Rest with the ACR stock brings the line of sight up perfect for an optic such as the Spectre DR (perfect on a 543) or for an LPVO in a cantilever mount such as the American Defence mounts. Here's my 543 with the ACR Buttstock:
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