I can't afford the whole rifle now, so I figured I better get something before it's too late. Placed an order for a CMMG lower w/lpk installed for $180. I saw some lowers for around $110, but then you had to pick up the lpk and have it shipped. Anyway, I want to get a collapsable stock, but I'm not sure where I should look and is there a certain style/model of upper receiver that I need for optics? Thanks...