I've seen polishing the A5 internals increase the velocity up to 10fps under the same settings. By removing the factory overspray on the receiver halves the theory is to improve marker efficiency by reducing drag on the A5 internals. One thing you need to keep in mind is that all you want to do is remove the overspray and polish the receiver halves without removing any metal and changing the internal diameter. Do at your own risk.

That said, here are the items I use to polish the internals of the A5.
Brasso Polish, Paint stripper, 00 steel wool, and a small plastic brush (purchased at my local hardware store). A clean wrag and some paper towel are needed as well.



The first thing I do is clean the receiver halves to remove any oil and dirt.



After the parts are dry, I apply a thin coat of paint remover. Remember to be careful not to get this anywhere but where you want the overspray removed from, I also recommend doing this in a well ventilated area.



I then let the parts sit a few minutes to allow the paint remover to work. You don't need to let it sit very long as the steel wool will to the real work.