The velocity adjustable bolt is also known as a 'dialable' bolt (what the Proline manual calls it). A dialable bolt cannot increase your shooting velocity higher than what you get from your stock bolt (unless your dialable bolt is more gas efficient). To increase your shooting velocity significantly, try changing springs or adding washers.
The dialable bolt I've got is bare aluminum and looks just like my stock bolt except that my dialable bolt has a brass piece down the center of the bolt. This brass piece obstructs gas flow depending on how far it is screwed in. In order to reach the brass piece and adjust the velocity, you need to slip the pump rod out of the bolt (which might encourage wear on the pump rod threads in the pump grip). There is also a spot for a tiny set screw to hold the brass piece and keep it from shifting.
I could try to find a set screw to replace the brass piece so that I could adjust the velocity with a long allen wrench down the barrel (instead of needing to slip out the pump rod).
The dialable bolt pictured above was given to me from a teammate at Skyball 2000, so I don't know if dialable bolts from Proline are exactly like this. I'm sorry, I don't know where you can buy dialable bolts. If anyone knows, please tell me. (Update: Coutel in France might have dialable venturi bolts [Note: Coutel's web site is in French.]. Check the Upgrades or 'Where to Buy' sections.)
You could also try to make your own dialable bolt by getting someone to tap a hole through a spare stock bolt and put in a set screw. You could tap the hole perpendicular to the bolt and by the feed port so you can adjust velocity throught the feed port instead of from the rear of the bolt. If you do install a perpendicular set screw, make sure it doesn't stick up out above the surface of the bolt, or the set screw will scratch the receiver whenever you pump the gun.
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