I'm certain this has been covered but I'm new here so have not seen it. I have a DPMS LR308, function is great, but I have a buffer question, what is the length difference between a carbine tube, and a rifle tube? Mine measures to roughly 6 7/8" on the inside, is this considered the rifle tube, or carbine? I am installing a MagPul stock and would prefer to be able to shoulder the weapon closer to get a shorter eye relief for my optics. I've mitigated recoil with a Hypertap brake, works amazing, so a closer eye relief for the ACOG scope would be helpful! Any answers?