I bought a e-bay trophy, its a colt hbar barrel
with no sight or barrel extenison , can anyone
give me some pointers on installing a new extension ?  

The extension controls the headspace.  You will need access to a lathe so that it can be timed to the existing gas hole.  Unless you got it for $15 or so, it'll probably cost a lot more to get it into working condition than buying a completed barrel.

Could you amplify that a little? I was thinking of putting a chrome or maybe a TiN extension on an existing barrel. Why the lathe? I can see how there might be problems with headspace as well as lining up the extension with the bolt and indexing it with the barrel and receiver, but why a lathe? I'd appreciate it if you could outline the procedure for us so we know what's involved.

Get a barrel extension.  Torque it on until you've got it lined up with the existing index pin hole.  Check headspace.  If headspace falls within safe spec (for a used barrel like this, it can close on a NO GO, as long as it won't close on a FIELD), then you're good to go.  If it doesn't, there's basically nothing you can do about it unless you can weld up the old gas port and drill a new one.

If the headspace isn't within safe limits, and you're feeling adventurous, you can do the following:

1) relieve the barrel extension threads on the barrel so you can torque the barrel extension on a teensy bit more, if necessary
2) continue tightening the barrel extension a bit at a time, checking headspace as you go, until you get the headspace within safe limits.
3) weld up the old gas port and sight base pin grooves (you can weld up the old index pin hole if you like, but I wouldn't bother)
4) time and drill a new gas port and index pin hole.
5) time and install the sight base, and drill new grooves for the sight base retaining pins.

The need for a lathe is to smoothly relieve the extension threads on the barrel, if necessary, and to clean up the excess weld from filling in the old gas port and sight base grooves.

When you're done, if there was a headspace issue, you've got a really nasty barrel throat/leade, but it should work all right.  If you were lucky and headspace checked out using the existing index hole, gas port and sight grooves, it's just like any other used barrel, I'd reckon.

Circuits is absolutely "on target".

It is really false economy to buy any stripped barrel regardless how good it is if the gas port has already been drilled.  After purchasing a new extension and the proper tool to install and torque it on to the barrel blank, it's just not worth the effort (labor hours/dollars) to properly index the new extension to match-up with the preexisting gas port hole and still assure the correct headspace once you get the whole mess properly assembled.  Plugging and redrilling gasports and sightbase taper pin holes is just not a good solution.

IMHO you have purchased a really good piece of tubing best suited for use as a prybar in your shop/yard.  I would suggest you go out and purchase a complete barrel assembly from a reputable supplier that's ready to just screw on to your upper receiver.

Thanks for all the info, in fairness I don't consider myself the typical "home builder"
I have been a machinist for over 25 yrs.
and have access to all the machines.
the challenge of building a good "shooter"
with my left over part in my motivation
So, is there any other "how-to" pointers you all could give me ???
 

1) plug/weld gas hole and extension pin hole.  Turn/grind to contour.
2) buy/make an extension
3) get Colt or GI GO/NO gauge.  Remove extractor/ejector from bolt for clean headspacing.  Commercial gauges won't work with a Colt chamber.
4) when HS ok, mill for extension pin (drill won't work as it will catch on plug/weld)
5) set rear sight to middle setting, install laser bore sighter, adjust front sight tower/post until it matches sighter mark.  Drill sight base for #3 taper pins.
6) Mark location for gas hole and mill to .063
7) install everything and test fire.

Did this once and won't do it again.