they didn't specifically mention the 308 :
"THIS IS A BULLPUP SHOTGUN STOCK KUSHNAPUP MADE EXCLUSIVELY TO FIT YOUR SAIGA FOR TACTICAL COMPETITION OF SPORTING OR HOME DEFENSE! NOW YOU CAN TAKE YOUR ORDINARY SAIGA-12 AND CUSTOMIZE IT YOURSELF IN MINUTES WITH JUST A SCREW DRIVER, THERE IS NO GUNSMITH REQUIRED!"
"IT ALSO FITS AND FUNCTIONS FOR SAIGA’S 20GA, 410GA, 7.62 AND .223"
http://www.kushnapup.com/