Hop-up Variants

Introduction
A short summary of different common hop-up solutions.

For illustrations and a more detailed explanation check out Airsoft Ranch's Ultimate Guide to Airsoft Hop-ups.

Traditional hop-up
While hop-up designs vary between different platforms the vast majority use the same principle of an adjustable arm applying pressure directly on the top of the bucking to push it down through the window/recess of the inner barrel. The the part of the bucking that is sticking into the barrel catches the bb and applies backspin, and by increasing the depth of protrusion you increase the backspin (i.e. "increase hop-up") because the bb encounters more friction when passing which leads to more spin.

It's common to use nubs, small pieces that sits between the arm and the bucking pushing it further and better controlling the applied pressure/contact surface. Note that there are a lot of different bucking designs, some are meant to be used with nubs while some are designed to be used without.

The hop-up variants do not fundamentally change how it works, only how the friction for the backspin is achieved. The one exception being that TDC designs utilize an adjustment screw straight above the barrel window rather than relying on transferring the pressure with an arm.

Flat hop
The original flat hop is based on the principle of maximising the contact area of the hop up bucking by removing any protrusions inside the bucking and adding a flat nub.

R-hop
R-hop is the same as flat hop except you add a patch, typically cut from a piece of silicone rubber tubing, to the window of the barrel. This means it's following the barrel profile and can make even more contact with the bb compared to flat hop. Usually requires you use a very thin nub to not over-hop.

S-hop
S-hop, or Sugru hop is the same as an R-hop, but you use Sugru (compound is actually called Formerol, like RTV silicone putty) to create the patch instead of using tubing.

H-hop
H hop is where pressure is applied more on the sides of the bb and less in middle, theoretically improving short to mid range accuracy without affecting long range performance.

TDC
TDC mods are worth an honorary mention even if it's not a modification of the bucking or nub.

It's an abbreviation for Top Dead Center and is a hop-up unit design that applies pressure on the nub from directly above, which gives more consistency and control compared to being applied with an arm or similar indirect means. Some guns/hopup units lend themselves to being modded this way using a grub screw, and a few are designed this way from the start.