I have a DPMS lower and two DPMS uppers that I built (first mistake).  Both uppers have other barrels but have DPMS upper receiver and BCG.
I have a 16" barrel with a mid length gas system and a 20" barreled system. I have a RRA trigger.  Nothing special.
Did the head space test and everything appears fine.  Both carrier groups have the same overall dimension.  Firing pins measure the same.  The bolt's measure the same.  I have not been able to accurately measure the bolt depth or the firing pin protrusion.
The 20" upper worked just fine.  The 16" upper went full auto for 3 rounds and jammed on the 4th.  Went back to the 20" upper and it worked just fine.
The range was closing and I did not have time to swap things around.  Wanted to swap the bolt's to see if that was the issue.
Am I on the right track looking at the BCG or should I look at something else?  
Thanks for any insight.

I don't believe an upper can give you that effect, it would have to be something within the trigger group I believe.  How many rounds do you have through the 20" upper with no issue?  Have you checked the hammer spring to ensure it is not installed backwards?

I put 37 rounds through the 20" upper on this trip without an issue.  I put 40 rounds through it on another trip.
I have checked the hammer/trigger group.  All of the springs are installed correctly.  I have installed 4 RRA triggers into AR-15's without issue.  I know that the RRA triggers are approved for use in the AR-15 but is questionable for use in the LR-308.

I would next check your BCG in the 16" upper, the firing pin could be getting stuck and slam firing.  I would ensure it moves freely and easily within the bolt, even add a little lube.

That is what I was wanting to do but ran out of time.  The range was closing.  I figured maybe firing pin protrusion causing a slam fire condition.  
Maybe next time.  I will lobe things up, load 2 rounds and try it again.

are you shooting factory ammo?

It sounds like the firing pin may be stuck in the forward position or the disconnector isn't locking to hold the hammer after a shot  and slam firing.

Did you "polish" or take material off of the disconnector to change it's range of movement?

out of battery would definitely not be a good thing if the bolt isn't locking before discharge.

I am using factory ammo, Hornady 150 gr. Interlock.  Cheep stuff for sight in purposes.  16" Brownells 308 barrel,  JP handguard, Aimpoint CompM3 on a American Defense QD mount on a DPMS upper receiver and BCG.
I did not modify or polish the disconnector or the hammer in any way.  Straight install.
I will look at the firing pin and bolt.  I did notice quite a bit of resistance between the firing pin and the bolt.  I do not have a small enough wire brush to clean the firing pin hole.
I am still trying to make sense of all of this.  If there is considerable resistance with the firing pin and the bolt, it should retard the forward movement of the firing pin.  Do not polish either part.  
The hammer ramp measures the same from the top of the gas tube base.  This should push the hammer into engaging on the disconnector.  
Maybe their is a clearance issue inside the upper receiver itself allowing the BCG enough clearance to where it is not pushing the hammer down far enough to engage the disconnector.
I will look at that angle.
Thanks for your input.

FIRST, swap the bolt carriers (not bolts) and see if the 16" upper gives you problems.  It can't just be the trigger goofing up with one upper and not the other, but the carriers could be different.  I'll put a whole quarter on it being a difference between the two carriers.  Is the carrier with the 16" upper shrouded or not?  I'm thinking not...

The 16" carrier is not shrouded.
It will be a while before I can get back to the range.  I was going to start by swapping out the BCG and work down from there.  The range is a 45 minute drive one way.

Remember, swap the CARRIER only; the bolt shouldn't be switched from upper to upper.  Yeah, it's probably not a problem and neither gun will have headspace issues with the other's bolt, but make me feel better about suggesting it and only swap the carrier.

The firing pin does float freely until you try to push it into the firing pin hole in the bolt.
Is it the bolt or the firing pin?  Both firing pins have some resistance when engaging the firing pin hole in the bolt.  It feels as though the 16" bolt and firing pin have a little more resistance when engaging.
I see a square hammer pattern on the 20" carrier that I do not see on the 16" carrier.  
Swap only the carrier and not the entire group.  OK, I will do that.  Keep each bolt matched with its barrel.

The firing pin hole is dead center on the 16" bolt but off center about .005 on the 20" bolt.
Both holes are .078".

Tore the thing down a dozen times and could not find any issues. Swapped carriers and did several function test.  Measured just about every surface and dimension I could get the calipers on.  Could not find anything out of tolerance.
Took it to a range, loaded one round into a magazine to see if it would hold the bolt back and reset the trigger.  It did.  I did that several times.  It worked fine.
I then loaded two rounds.  It worked fine.  Did five rounds without an issue.
Something did not mesh during assembly.  Tearing it down and reassembling it several time put it right.  I have no clue.  One of life's mysteries.  
Anyway, thanks for the help.  

I would look at the disconnector spring being upside down

Disconnecter spring needs to be large diameter curl down, narrow curl up in the trigger.

Also completely strip the bolts and spray them out with Gun Scrubber of a similar stripping agent. The firing pins shouldn't be tight or "sticky". You may have dried metal preservative built up or WD40 is notorious for this. Never use WD40 on firearms without completely stripping it afterwards and adding lube (light oil). WD40 turns into a varnish over time, very sticky.