After a bit of a goose chase on how to get a Surefire Scout light onto the Sig SG 551 we found the solution with the Haley Strategic Thorntail.
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The light and thorntail were mounted to the handguard using a Magpul MOE polymer 7 slot rail
This setup saves on weight over the alternative aluminum handguards and it keeps the flex that the original handguard was designed to have. With the handguard fixing into the gasblock, and the gasblock being attached to the barrel the flex is designed to prevent the polymer handguard from effecting point of impact downrange. The aluminum handguard would attach directly to the gas block through the bayonet lug which effects point of impact downrange far greater as it applies pressure to the barrel through the gas block.