Hi there!
I have replaced the barrel on my Maruzen M870 with a metal barrel. Mind you, this only works for the barrel, I believe the mag tube is too wide to be fitted. However, the mag tube on the Maruzen is metal already. I got the "GB-Tech-M870 Metal Barrel & Mag Tube Set-10 Inch". The barrel did need some modification though:
- The barrel does not fit over the metal "funnel" at the front of the Maruzen, so you'll have to dremel out just a bit of metal from the inside of the barrel. It's not much though, so no problem.
- You can attach the barrel with the grub screw at the top, and you can also tap the little holes on the side to add more grub screws. Mine surely won't fall off now.
- Lastly, when cutting the barrel down to Pulse Rifle specs, you'll notice that the metal is really thick and the inside diameter of the barrel is pretty small. I simply used my dremel to widen the inside diameter, took me a while though.
Hope this helps.
EDIT: Oh, I also have the "G&P M870 Shotgun Metal Body" and it fits perfectly. I had to dremel it a bit to fit it into the SPAS cage.
Cheers!
Roman