[Surestick]

The velocity will be adjusted in accordance to AGD specs

I assume they are all cocker-threaded barrels to be tested on a mag.
Have you thought of trying a few different markers to see if there is a better barrel for a specific marker/valve style? Mag and Cocker should give you spool & poppet valve but any other cocker threaded markers you can get your hands on would be good too.

Another thing that might be worth keeping track of is efficiency. See how much the velocity increases or decreases with the same paint & a fixed velocity setting for each barrel.
If you are testing different markers it might be interesting to see if a specific unported length gives a better efficiency w/one valve style vs. another.