Good advice, and very, very easy to do.
Dremel's own "Chainsaw Sharpening Bit" is the bit to use. Straight stone, very small in diameter. Easy to use to blend the lower to those wide feedramps, in this/your case. You'll be removing softer aluminum, and won't damage the harder barrel extension. Polish afterwards, easy job completed.
Yes, I saw that you came up the the ejector cure - blend the lower and feedramps anyway... <thumbsup>