I am not aware of an upper that is available for purchase to create a semi auto version of the Colt LMG.  (If there is one, point it out to me please)

If I wanted to build an upper that recreates this as closely as possible what would be the best way to do it?

I know I need a heavy barrel.  (1" diameter I believe)  Also a thicker gas tube.  Where would you source the parts and how would you go about this build?

I know it originally fired from an open bolt but I am sure this build will have to be closed bolt unless there is some way around this.

http://www.autoweapons.com/photos/accys/colt.html

Don't know if I should hot link to that but there you go, they want $3395 but even if you didn't buy it the pictures might help you get it right.

What's the story on this barrel?

If you really want one, your best bet is going to be to jump on this:



http://www.gunbroker.com/item/568393757


Piecing one together is not impossible, but it's going to take a lot of time and a lot of frustration, and will almost certainly cost more than simply buying this outright.  

If you insist on trying to find parts and "build it yourself," TACLTC on Gunbroker will be a good source for some of the more eclectic parts, send them a message if they don't have something specific listed, they might have them, again, expect to pay, however, you're talking about collectors' parts, collectors' market, and collectors' prices that are totally unhinged from the practical value of the items.  

Again, I can fully understand the reasons someone might want to make a vismod version of the LMG (though that particular model has never particularly appealed to me) in semi-automatic, but understand that if you're expecting any kind of increased functionality out it in a semi-automatic configuration, you'll be sorely disappointed relative to the cost of the project.  

In semi-automatic only, you can functionally build the exact same thing with a Colt Match Target HBAR upper and a bipod.  

~Augee