Is it blowback? If you used a different gun then it would be pretty cool, as you could be like "Its a suppressor"
(sorry, Im gonna be a gun nerd and say that a colt M1911 could not be silenced as the .45 caliber bullet it fires is just too much of a power house, and it would take away from its distinct bark.)
But say you did this with a MK 23. that would look sweet as a dart conversion, and the cover for the PETG would make an awesome suppressor like device, even if it wouldn't work.
Waste of an airsoft gun, but it looks pretty cool, and I bet its fun to plink around the house with.
Props
Isn't the MK23 also .45 caliber? Also, I've definitely seen pictures of suppressed M1911s, but I'm pretty sure they were firing subsonic rounds.
On topic, I think that you definitely need a longer barrel on that thing to maximize range. I would play around with different barrel lengths until you find the most effective one.