I just got a Rock River Predator Pursuit which had been fired a few times and customized some and came with a Magpul CTR stock. It is too short. I can't get proper eye relief on my scope and am just kind of scrunched up. Is this a carbine length tube? and will I have to put on a rifle length tube to install a PRS stock? I think the AR15 stocks are different from the PRS I have on my LMT MWS. Thanks if you can save me a few steps...