I've found that the OD of the sprocket carrier, and the ID of the actual rocket, have a actual difference in measurements. There are tolerances you have to consider. If the tolerance on the carrier was too much, and didn't match the sprocket, then what. You always have tolerances when machining anything. I don't know what the tolerance is when machining sprockets and carriers, but you don't want the sprocket to small or the carrier to big.
If you test the tention on the chain, as you turn the axle, you'll see that in some spots the chain is looser than in other spots. I always turn the rear sprocket until the tension on the chain was the highest, then, using a 2 x 4 up against the rear of the sprocket, I would hit the 2 x 4 with a hammer. This always worked for me.