Is there really a difference in Colt's lower receivers 6721, 6921, 6933 in terms of their performance?  Aren't they essentially the same?

Also, other than full automatic/3 shot burst capabilities, is there any other difference between RO977/9 (military version) lower receiver and law enforcement lower receivers in terms of their performance?  If there is a major difference in spec other than full auto/3 shot burst, I would like to know.

Just very curious to know.

Most any lower receiver will perform as well as any other.

There is more material milled out of the FA receivers to allow the installation of the auto sear. Other than that there is no difference between pretty much all forged receivers on the market.

I guess the question is then why does SOCOM issue heavy barrels as opposed lighter standard issues.  Any thoughts?

The "SOCOM" barrel is a result of specific tactics utilized by SOF units, particularly when breaking contact.  

SOCOM's M4A1 is fully automatic, rather than "burst," and was often used in that capacity, and the "wasp waisted" standard M4 barrels were sometimes bursting when used in sustained fully automatic fire, so SOCOM requested that the barrels be beefed up to prevent bursting.  

More specific details can be found in The Black Rifle II, but that's the gist of it.

The original M4A1 Carbines were identical to M4s other than the FCG.  

RO920 - USGI M4 Carbine with S-1-3 FCG

RO921 - USGI M4A1 Carbine with S-1-A FCG

RO921HB - same as above with "SOCOM" profile barrel

RO977 - Commercial/export sales version of M4 Carbine, usually with S-1-A FCG though a burst FCG can be installed, and for all intents and purposes identical to the RO921, other than the rollmarkings.

RO933 - 11.5" barreled version of the above

LE6920 - 16" barreled version of the RO977 with S-1 FCG (semi-only) and larger FCG pins.  Lowers built with "web" to prevent installation of automatic sear

LE6920HB - short run commissioned by Ken Elmore of SAW, limited to 300 units, identical to LE6920, however, the barrel has a "SOCOM" style HB profile under the handguards

LE6921 - SA version of the RO977, lower is identical to LE6920 with exception of rollmarkings.  14.5" OAL barrel causes this carbine to be classified by BATFE as a SBR, and therefore regulated under the NFA

LE6921HB - to my knowledge, no factory examples exist, though hypothetically speaking, they could be built.  Any LE6921s with HB barrels have most likely been modified after the fact, however, if these were ever offered, I would be extremely tempted

HTH,
~Augee

Quoted: I guess the question is then why does SOCOM issue heavy barrels as opposed lighter standard issues.  Any thoughts?

No disrespect but what does barrel profile have to do with your receiver question? Medcon SOCOM barrels have to do with the M4A1 (full auto) verses the lower rof M4 (burst). The lower has nothing do with the robustness of the barrel profile other than containing the fcg parts that fascilitate that rof.

Sorry about the confusion about the barrel.  I had two string of topics going, and I posted the barrel question on the wrong topic.