Have been working on it some more. Made an encapsulated front part, for silencing and rubber protection purposes.
I still have to attach brushes to the front and back holes, but it already is very quiet. I also might include a band heating system.
Equipped with 3,1cm x 3,1cm bands (four strips per side), it shoots the 40 gramm aluminum arrow with mighty force. The field tips penetrate about 3cm into massive oak panels. The field tip is pushed down the shaft in the process.
As happy as I am with the functionality, as unhappy am I about the looks. Boxy, clumsy, foolish. Back to the drawig board.