I searched and didnt find an exact answer to my question.  I am building my first AR and am going with an 18" barrel using a mid length gas system.  I am thinking of using the Spikes 6 position buffer tube assy that uses the ST-T2 buffer which is right about 4oz.  Will this work?  I was reading it may be too heavy.

I have a T2 in a Spikes 14.5" mid-length and never a problem.

The buffer weight is only one area of concern. Gas Port sizing, dwell time and even the  type of ammo also will affect proper function.

Your dwell time would be longer in that barrel so I would say if the gas port is sized correctly then you should have no issues.