In the case of a precision made air rifle, such muzzle devices have proved to make for a slight increase in accuracy. Edit: They're known as "air strippers", kinky huh? actually, here's a one, VERY kinkySpudBlaster15 wrote:3) Porting the barrel is thought to increase the range and accuracy of the projectile by preventing the gases escaping from the end of the barrel from pushing the exiting projectile into a tumbling state.
In the case of spudguns however, where you're firing something that was plucked from a field, the accuracy benefits are dubious at best. If your projectile is unrifled, ported barrel or not it will still start to tumble the millisecond it leaves the muzzle. Hell, even rifled projectiles start to tumble, but the gyroscopic effect of the spin tries to bring them back, but they then move too much and start to tuble upwards, and again the gyroscopic effect tries to correct this - the phenomenon known as "yaw"
How Do Bullets Fly? - a well written short study, worth a browse through
I don't think you can really quantify any advantages other that "it r0xx0rz 2 shoot @ nite!", and I guess it looks pretty cool too. It could also be argued that the lateral flow of propellant gas helps to reduce recoil (though not as much as it would were they to be vented backwards towards the shooter)
Also, shroud the ported area well and you have a silencer
