It's the tube that is ruining your day. Also I would change out the buffer and spring for proper 308ar parts from Clint at Heavybuffers.com. I run his heavy stuff on all my ar's, big and small, including my OBR. You will decrease recoil and follow up shots will be much faster, and yes, you can keep the SOPMOD.
Here's a reference chart from his site, and you can see the tube is definitely off.
http://heavybuffers.com/reference.html