First things first. You'll need a fixed barrel adapter. Then you can Amazon a 5/8 to 1/2 adapter
http://www.midwayusa.com/product/121326/advanced-armament-co-aac-fixed-barrel-spacer-ti-rant-9-eco-9-evo-9-suppressors-aluminum-black
http://www.amazon.com/24-thread-adapter-308-223/dp/B00V566TXU