Thick rubber and silicon hopup buckings, and great spacers. Unfortunately, if you want to use this on a standard AEG, you may have some issues fitting the bucking into your hopup without some... modification.
I ordered a G&G metal hopup for my CA Aug at the same time as this. I spent several hours trying to get either of the buckings in to the hopup correctly, and despite using lots of lubrication and effort, got nowhere. The best results I managed were when I took the rubber bucking and cut it down to half its current length, reducing the friction that was stretching and deforming the bucking as I slid the barrel in. Now it's in and the gun shoots VERY well.
Buckings are thick, and should take some abuse.
Spacer should work quite well, distributing pressure on the BB evenly for more accuracy (haven't tested yet at long range, but short range seems very consistent).
May increase FPS slightly due to better air seal.
Major pain in the ass to get on.
I wouldn't recommend this for most people, as it wasn't really designed for AEGs to begin with. Get yourself a similar type of spacer, but a more conventional bucking that isn't quite so thick.
Rating: [4 of 5 Stars!]