In combination with an Action Army VSR Hopup chamber,you can realize an external hopupadjustment.
Additionally you will need skeee's TDC base(aviable here) and a spring loaded locking pin (M3x7) as well as a special M3x17 screw with an extra small head.
For more information please check:
http://www.airsoftsniperforum.com/41-vsr-rifles/15600-aa-hopup-unit-external-adjustment.html