For the OP, as was noted earlier, make damned sure that the flash hider is either pinned or welded; putting a setscrew in that hole and then getting a spot of weld on top should be enough to solve the problem.
I'm inclined to agree with the Judge and assume the barrel has been replaced. Is it chrome lined? Again, it's my impression that Colt barrels were marked with caliber and 'chrome lined', if they indeed were. The upper has the right markings for Colt production.