Ed, how did you get it to fit, and would it work for an M4 style hop unit?
Well, before I put it in my L85 I had it in my G&P M16 for a bit whilst I was waiting for my L85 to get back from the shop and it just worked right off the bat.
Standard AEG hop rubbers have a groove along the bottom inside edge though and the female groove in the barrel is a lot shorter, so it doesn't mesh 100% with hop rubbers, but they do work.
It also doesn't mesh with the lugs that hook into the grooves on the side of the barrel to hold it in place, so you can either leave the retention bit out, as it doesn't really help much, or you can just put it back in anyway, though it won't fit properly. I held the barrel in place by wrapping tape around the outside of it to create a friction seal between the inner and outer barrels. Once it was in there it worked fine, really made a properly noticeable difference as well.
One more thing though, the outside diameter of the barrel is wider than the stock barrel, meaning it's quite a tight fit in the hop unit, I had to use the stock rubber as the madbull one is slightly thicker and wouldn't squeeze in.
As for making it fit in my L85, I just cut the madbull hop rubber in half so that the groove along the bottom inside edge did mesh properly with the barrel, then removed the standard L85 retention piece and then did the same as in the M16 to hold it in place, just wrapped tape around the barrel until it fit very tightly into the outer barrel.
Works a dream. Hop is barely on at all and the gun shoots at least 45m before there's even a hint of pellet drop.