I have an AR pistol lower.
To be clear, if the flash hider is not welded, brazed, or pinned on the barrel, it's an illegal Short Barreled Rifle (SBR).
That means it's a Felony crime to have it, full auto or not.
Often these were pinned on with a blind pin (hole wasn't drilled all the way through) and the pin was welded over. Often the weld and pin are hard to see.
Other times the flash hider was silver brazed (silver"soldered) on, in which case it can be difficult to see. Look for a trace of silver around the area where there hider joins the barrel.
Also they could be welded on. Look for a bead of weld.
The BATF has strict standards on how pinning, welding, or brazing must be done.
Brazing has to be done with a true brazing compound that melts above 1100 degrees.
Welding has to have a bead of weld a minimum of a certain distance around the hider. I can't remember but I "think" it had to be at least 1/3rd around.
Pinning had to be in a blind hole and welded over, or possibly all the way through and welded over on both sides. I'm not sure about this.
For your own protection it would be a smart idea to try to figure out what the situation is with the flash hider.
I have an AR pistol lower.
Personally, I would try to find an original Colt 16" light weight barrel and return the upper to the original config. Brownells & Doublestar sell LWT barrels with front sight towers installed (A1 barrels). But I'm not sure how handy you are at pulling a barrel from a receiver. I'm sure theirs plenty of videos on line. And all you need is a receiver block and barrel wrench (available from brownells or Midwayusa).
Here's the last Doublestar LWT barrel in stock at Midway. And it's even chrome lined.
Watch & Pray
I think that's the way I want to go. Maybe hold out for the Colt barrel. How common are they on the market? Going price when found? Worth the extra effort and expense?
I've never really paid much attention to prices of these, but here's what I found on Gunbroker. There's a 1/12 twist Colt barrel right now for $350 (not sure what your rifle should have). There's also a complete large pin upper (you could pull the 1/7 twist barrel & sell the rest of the parts) with 1 day left on auction for $300. And a few 6720 (1/7) take off barrels from $150-240. But these will probably be "F" marked sight towers. Not really what you want.
You can also keep an eye out on ar15.com in the parts section.
Watch & Pray
I had a Colt model R6530 Sporter Lightweight carbine that I bought new in 1991, I used it for a very long time but sold it during the ban in 2004 for a very good high price.
My carbine's factory configuration was an early version, the early version had 5/8 inch shorter of the A2 solid buttstock with same trap door/ storage compartment, after the early short stock type they went to the standard A2 longer buttstocks.
These carbine model were made from 1990 up to 1994 and they were replaced with the model Match Target Lightweight MT6530 in '94 and later, the main difference are the post ban Match Target series had no flash hiders.
My barrel was a 16" A1 type pencil barrel 1/7 twist with A2 flash suppressor, the bolt carrier rear bottom end had been removed (the back end looks like a "C" shape), it had a steel sear block above the selector lever, and the front pivot pin was a screw type (not a mil-spec captured pin).
Your lower half looks fine the steel sear block had been removed and your buttstock replaced with collapsible carbine stock, no need to change it back to a solid fixed stock (unless you want to).
The upper you could replace with another pencil barrel, Colt or other brands, check out Brownells for retro type carbine barrels, and I agree also look at AR15.com Equipment Exchange section for parts.
Colt complete guns and components had gone up quite a bit recently because of the factory's announcement that it will no longer make civilian models for several years until they could catch up with their military contracts.
I just received my panic buy of a Colt LE6920-EPR (Enhanced Patrol Rifle) complete upper receiver half, 16" standard gov't profile barrel, low profile gas block, free floating Centurion Arms 13.5 M-LOK modular rail system, factory kit in a blue and white box.
I will post pictures of it as soon as I put backup iron sights on it.
NRA Life Member since 1973
Colt AR-15 Sporter SP1 rifle owner since 1971
Colt AR-15A2 Sporter II carbine owner since 1985
Colt 1911 Government Model Series 80 .45 pistol owner since 1984