Well, I tried it and it didn't work as planned. I like your idea with retaining stock aesthetics so instead of putting it on the outside of the shell I kept it inside the shell but outside the plunger. It would have worked except that there wasn't enough room for the springs I had. It may work with different springs or a different shell.
Maybe it would work better if you took two smaller springs, equaling about the same strength of the one you used. Then took a bolt and put it through the tube that runs from the o-ring to the barrel like you did, and put another bolt through the plunger tube. That way you could attach them to the outside, and take up less space inside the plunger tube. You would also have to make sure you seal it up so no air is coming through. There may be slight issues with the stock spring but I'm sure with a little tinkering it would work.
Hey guys I'm new to Nerf Haven and have only done a couple simple mods so maybe I'm wrong but, wouldn't putting the spring inside the plunger tube decrease the volume of air inside the plunger tube which would lead to lower ranges. Obviously it increased ranges from just the normal AR removal but maybe you could place it somewhere else and that might boost ranges a little bit further. Also, are you using this mod with the stock spring? Thanks.