What's the use of that little metal ring around the inner barrel?
 

It's past the hop-up unit. It's thin and about 2mm long.

I don't recall ever seeing one on my TM M4A1. But there was one on my CA G36K. I changed for a Systema tight bore, and now the ring doesn't fit on the outer diameter of the new inner barrel.

Is it a little metal ring? It's to stabilize your inner barrel with your hopup to minimize movement/rattling.

Ok, That makes sense. I added layer of tape around the new inner barrel to stabilize it, but I did it before with the stock inner barrel too.