Built a 308 with a Sharps Bros receiver set with a BallisticAdvantage mid-length barrel, 8.5oz buffer and carbine buffer spring from heavybuffers.com.

Unfortunately, I am having a problem with short cycling.

The second round is not being stripped off the mag after firing.  As a test, I tried loading just one round in the mag and the BCG will not lock back on the empty mag after firing. 

I also swapped the heavy buffer for a 3.7 oz buffer.  Worked a few times, but which mostly had the same problems.

Since the lighter buffer did not work, I am thinking one of two things  1. buffer spring is too heavy   2. Gas port needs to be opened.

Thoughts and suggestions?