Muzzle velocity question...

I just traded a Cetme for this PLR-16 with extra long flash-hider. Since I've never owned a weapon that didn't have the darn thing welded on, I don't know exactly how the flash-hider works as a barrel extension. Since the ports are at the end of the hider, (see picture) can I assume that this will add another 3-4 inches worth of muzzle velocity to the gun? Or is it not a good enough seal with the barrel? Just curious. I'm debating whether to keep it or just buy a Bushie muzzle brake.

http://i15.tinypic.com/29zv4p0.jpg

I have the same flash hider on an M4gery, it works well, I've tried it at night. It however deos nothing to add to the velocity. It mainly is in place to meet barrel length restrictions. I have a PLR-16 also, the muzzle flash and jump are pretty bad. I would geuss thats why the previos owner added the hider. I would replace the long one with a compensator since you don't have to meet any barrel length restrictions.