Completley normal, mine was doing the same thing for the first 500 rounds. I took some very fine sandpaper and steel whool and started to sand down some of the paint on the inside, when it was really smooth I would take the steel whool and make it smoother. There are still large paint areas on the inside, but its smooth enough that I have not had any more chip off in 2000 rounds.
Again, its normal, don't worry about it. If you really really want, take some paint thinner gel and coat only the inside of the reciever, when the paint turns into a liquid, whipe it out with steel whool, make sure that you don't get any on the outside of the reciever if you still want it black. Take a paper towl after and dry out the inside and there should be no paint left anywhere, when thats all said and done take it to the sink and wash it out with dish soap and water with a brush.