If I am understanding you correctly, the bushings (which are NOT brass) are not fitting into the brass tubes.
If this is correct, the highest probability is that you have glue in your brass tubes. When glue dries, it is clear, so you can't see it.
Use a penknife or a round file and rough up the inside of the tube, you will then SEE the glue--remove it. The bushings will, once again,
fit.
If, instead, you are saying the hole in your bushings is too small, try other bushings or "7mm" tubes. IF they also don't fit, sand down the mandrel some--not all mandrels are exactly the same diameter and "7mm" refers to the size of the hole you drilled, not the size of the tube.
Hope this helps,
Ed