Several weeks ago, I was having some compression problems with my M4 AEG airsoft gun. With no time on my hands to troubleshoot it, I took it to a local airsoft store to have it fixed. The gun shoots beautifully after getting it back. Once I had some time, I took the receiver apart and found a Bravo Hop Up Chamber (see photo below).
The Bravo Hop Up Chamber is made out of aluminum. Unlike most other metal chambers, it has a rubber O-ring seal at the back of the chamber (see photos). It seals the hop up chamber to the front of the gearbox.
When I took apart the M4 receiver and pull the gearbox away from the upper receiver, the hop up and inner barrel came out with it! I never seen that before. The seal was so strong, I thought the Bravo Hop Up Chamber and the gearbox had fused together.
The Bravo Hop Up Chamber is inexpensively priced at the low end of the metal chamber spectrum. But the product is very high quality and I have been very happy with its performance. Recently, I have found the Lonex Airsoft Enhanced Hop-Up Chamber for M4/M16 AEG. The Bravo and Lonex hop up chambers seems to be identical.
Amazon product page for the Bravo Hop Up Chamber and the Lonex Hop Up Chamber have exploded views of all the parts laid out separately. If you want to see all the parts, take a look at the page. It's linked in "Related Links" below.