Quick video from Stalker Airsoft Designs of their Kraken TAC-41 TDC Hop-up Chamber... "Convert your TAC-41 with the Kraken TAC-41 TDC hopup unit! A true TDC hopup unit which uses GBB/VSR style buckings, the favorite type of buckings used by airsoft snipers.

The design of the Kraken Hopup and Kraken TAC-41 Cylinder head is made in such a way that the BB is perfectly placed in the right place on the bucking patch, giving your TAC-41 the most consistent hop, resulting in more range and accuracy. Next to that the cylinder head comes with 2 bumper pads, this reduces the sound of contact between the cylinder and hopup unit. Making your TAC41 just a bit quieter.

The design of the Kraken TAC41 hopup unit makes it impossible to misalign the hopup in between hopup unit and bucking/outer barrel. The hopup unit has 2 cutouts that work perfectly with the Stalker TAC41 Double Fin SR-hop buckings. The complete hopup avoids any wobble with the square barrel spacer and 2 locking screws. The hopup main body has a raised edge under the wheel to avoid any water entering the TDC nub compartment."