Note: The PRS AR10/SR25 stock may be used on AR15 / M16 rifles to provide additional charging handle clearance. Stock is not compatible with Bushmaster BAR-10 and Rock River Arms LAR-8
RRA website suggests calling before trying to use one on an LAR-8, but the reason is because the cheek piece doesn't give you enough room to charge the weapon. Take a Peek
The two ways to fix it are either cut a small piece out of the front of the cheek piece...(buffing it smooth and you'd likely never even notice)
OR, get a stock extender. I don't know if that's a very good option for a rifle length stock unless you got really long arms.
Here are some pictures of what I believe is a RRA developed "mod kit" for your PRS if you want to use it on a RRA LAR-8. Call for price and availability, last I heard was around $25 extra.
thx cbass16 ...have to wait till i get home to hit the rra site ...crazy comps at work block nearly all firearms sites ...but thats good info ...really been wanting that stock for my gun ...glad to see there is some work around for it.