Pretty sure the breechface and extractor would need to be different too due to the smaller case diameter. So you would need to replace the whole upper portion of the gun. The recoil spring from the 9mm would probably work but I would want to replace that too in case it needs to be tweaked for optimum reliability.
But since the serial number is on the FCU you could at least buy all of that without going through a FFL even though it would be more expensive than just a barrel and magazine. And if you are married to someone who mistakenly believes you have enough guns you could point out that legally you didn't buy a new one, just modified an existing one.
But since the serial number is on the FCU you could at least buy all of that without going through a FFL even though it would be more expensive than just a barrel and magazine. And if you are married to someone who mistakenly believes you have enough guns you could point out that legally you didn't buy a new one, just modified an existing one.