I got to fully tearing down my recently purchased preban Colt A2 Sporter Target (ST prefix, preban S/N)to give it a complete cleaning after the first time I fired it. Note that I believe that this was rebarreled with a 1:9 tube since many have commented that it should have a 1:7.  When I fired a few 20 rd mags of SA M193 at 100m targets, I was getting between MOA and 1/2 MOA groups.  Only my Swedish Mausers and my ancient Remington Mohawk .222 shoot this well.  Others in my club with ARs get mostly 1 to 1.5 MOA.

When I bought the ST, I noticed that the front sling swivel was attached to bosses on bottom of the handguard front retainer cap, instead of to the bosses on the bottom of the cast front sight base behind the bayo lug and in front of the handguard retainer.  WTF?  I don't remember this from Parris in '72, but we had the A1 at that time and this is an A2 arrangement.

Upon removal of the handguards I find a robust steel tube enclosing the barrel, with slots milled at top and bottom, and the tube is stoutly brazed to the star nut at the D-ring end, and to the handguard front retainer cap.  It appears that the HBAR 1:9 barrel full-floats with this arrangement, and it must have been fitted by the previous owner at the time of the rebarrel.  I assume that the accuracy has something to do with this.  

I'm like totally ignorant of ARs except for '72-'74 with the A1 the USMC was gracious enough to allow me to use.

Your informed and expert input on the apparently optional hardware found under the handguards on this upper is very much appreciated.  Is this brazed tube a standard A2 thing?  IF not, who made it, who installs it, how much does it cost, can one expect sub-MOA with a floating barrel?  TIA!

BRS

Yep, that's a CMP/NRA legal free-float tube for service rifle comp.  They are very helpful in accuracy shooting, especially with a sling.  Most major manufacturer's make them, but there are better and worse.  On of the foremost HP smiths posts over on the competition forum, you might want to post your question there.  Generally cost $100 - $150, definetly a custom job.

HTH....SRM

Thank you very much for your time in responding and for the info.  That explains why it groups well, and confirms my suspicions.

BRS