Robert, who made this beautiful custom SIG 510 / Stgw 57 / 510 / PE57 / PE 57 7.5x55 GP11 Sturmgewhehr, explains the design choices that went into it. Both Chappie and Bloke shot it a while back, and it is amazingly smooth.

Gruenig and Elmiger can't be linked to due to YT reasons, but I'm sure you can find their website :)