I've modded the hopup on 4+ M4's so far.
Cut/sand the nub...obviously evenly/nicely as possible. If you can shave/cut/sand it and retain the "cup" shape...let me know, I wasn't able to
I pick the rubber ring off the barrel...flip it inside out and place it back over the barrel to work on it. Holds it in place/shape good enough for me.
That setup then typically underhops...so I made a punch that I can use to punch out tiny rubber disks that I can fit into the hopup hole...then I use the ball bearing. Reassemble normally. (NOTE: so far as I know...this is similar to most of the "Stinger Hopup" setup...although that hopup rubber is custom molded).
I aim to get just the tiniest bit of hopup in the barrel with the hopup off...turning the hopup on will put more than enough on.
I also use 0.28/0.30 BBs...and they need more hop than lightweight bbs.
Seems to work well so far.
Tys