I tried this out to see how they would fare against my stock ICS M4 hop up bucking. First of all, these buckings are slightly thicker than SystemA's and provide better seal than the aforementioned.
A few numbers with an ICS M120 spring and JBU 6.03 mm tightbore barrel:
ICS stock bucking: ~410 FPS
SystemA bucking: ~380 FPS
Madbull bucking: ~410 FPS
Classic Army bucking: ~410 FPS
I repeated the test with the SystemA because I was surprised about the low FPS, but the numbers were confirmed!
One major drawback of the Madbull bucking: they are *too* long, and I had to cut off about 1 mm with a scalpel at the end of the rubber because otherwise it would interfere with the plastic tab that needs to snap onto the barrel. You'll know what I mean once you use the bucking.
I have fired several thousands shots with it and the bucking seems to hold fairly well. The hop up spacer/nub is shaped like an "H" and is made of rubber/silicone. Maybe it's just my personal perception that they improved accuracy a little bit over my stock spacer. But obviously there's nothing to "wow" about.
FYI, the clear cylinder that you see in the picture is only there to keep the bucking from collapsing during transport -- nice feature that even SystemA doesn't offer!
If it weren't for the wrong dimensions, I would have given this product a 5-star rating. But as I said, you have to shorten the rubber in order to make it fit. Therefore, only 4/5 stars.
Rating: [4 of 5 Stars!]