A great Blowback Unit, we have personally been running these for years.

Unlike the B02 (Piston Head Version) these come with O-rings which are far easier to achieve good seals to your nozzle. 

We use these with the and work flawlessly

Will fit 5.1 and 4.3 variants, includes 3 different size O-rings

Don't forget to pick up some spare too, you will need them - trust me!