Gus Norcross, originally trained on National Match rifles and pistols by the National Guard Marksmanship Training Unit, specializes in Garands, M14s and 1911s at his small shop on the coast of Maine. His website is www.angusarms.com. He will be offering gunsmithing tips and tricks on Fridays.
The Colt 9mm carbine is a simple blowback design with no gas system or locking lugs. Theoretically, not much can go wrong. Rather than designing a whole new type of lower receiver, Colt adapted 9mm magazines to a 5.56mm mag well with a roll-pinned adapter. The ejector is a sheet metal prong installed in this adapter. Occasional ejection problems can arise when the ejector becomes bent outward so ejected cases don't impact on the ejector face correctly as they are extracted from the chamber.
This is an easy problem to solve. Grab the ejector with a pair of pliers and adjust it inward toward the bore centerline slightly. A slot in the bottom of the bolt provides clearance for the ejector and if you bend it inward too far it will rub. Check to make sure there is no contact with the bolt by hand cycling the action while observing the bolt face or by viewing through the mag well.
Colt 9mm carbines
Colt 9mm carbines are equipped with a magazine adapter that is secured to the lower receiver with a trio of roll pins. This positions the magazine.
The ejector is a sheet metal prong positioned behind the magazine and located in a slot in the magazine adapter. It slides through the bolt head.
Normal ejector position
Normal ejector position, just behind the left-hand cartridge in the magazine, can be seen here. The ejector needs to be parallel to the bore.
Large slot in bolt bottom
A large slot is machined into the bolt bottom for ejector clearance. For best results, the ejector should not rub against the inside of this slot.