[MaciekA]

Also note that the seal is achieved when the tappet plate has moved the nozzle's base close to the front of the nozzle. It's very difficult to stage this by hand, since as John said you have to be moving your piston at a high speed to simulate the expansion of the main o-ring. If you have a nice vice , you may be able to secure your gearbox and cylinder such that you can hold the piston with your left hand and the end of the barrel with the other hand, with a piece of loose tissue near the hopup feeding tube, and test compression that way... But it's usually overkill to do this kind of testing.

The reality of airsoft manufacturing and inter-brand tolerances is such that you are going to occasionally have to experiment to find the best match between parts. A lot of guys in airsoft trot out the old nonsense like "matching brands is the best way to go", which ignores that a lot of stock parts (matched by brand) are often inadequately matched to each other or sub-optimally designed in the first place.

The only way to achieve really good builds with excellent compression is to study where leaks happen and have many slightly different spare parts and bits in your workshop to iterate and continuously test what improves or degrades a given compression solution.

Over time you'll be surprised how much variance there is.. Some nozzles bind on the cylinder head because they're too tight, or leak because they're too loose. Some are loose on their tappet plates. Some have the wrong taper shape on the end. Some are too long or too short, leading to terrible feeding or no compression. There are maxima and minima in every mating or interaction between parts in your system.

And some just won't work no matter how awesome they seem to be because the BBs from the magazine are preventing them from returning to the front fast enough (leading to the "cut 2 or 3 coils off the tappet spring" fix).