[Blackthorne]

It has nothing to do with compression.

Compression = Seal

Non Ported Cylinder = more air down the barrel

If your barrel is bedded properly, then what you are experiencing is "barrel suck". The bb isn't at the end of the barrel when the cylinder is full forward and all air has been pushed out of it, there is a tiny back vacuum when the piston starts its return trip. THis is what is likely ruining your groupings.

Full cylinder = air movement sooner = bb out of barrel before the piston starts its return.