Alitube 9mm SMG early prototype in blender

So im designing a compact 9×19 para full auto SMG, lets call it "Alitube 9mm SMG". (just making things in blender, not going to build it in real life because thats highly illegal). The receiver is 25x50mm steel tube with wall thickness of 1.5mm, making the id 22x47mm. The bolt is made of aliexpress 22mm OD cromo tubes and the barrel is fgc style ECM barrel. Things i borrowed from other designs are the lower of hybrid square tube, printed glock / menendez mag, fgc ecm barrel etc. I need help in figuring out what type of recoil springs im going to need, how many coils, wire dia etc (bolt guide rod is 8mm dia steel rod), how long area does the receiver need for the spring etc, how should the back plate / disassembly work. Any input will be appreciated.

https://i.imgur.com/gmHKMRA.jpeg
https://i.imgur.com/p8hlyEt.jpeg (bolt)
https://i.imgur.com/aaVVGah.jpeg