porting barrel

[3strucking]

I have a few questions on porting a barrel.

1. Is there a formula to figure out muzzle velocity based on diameter of ports?
2. Does it matter if some ports get in to the lands?

[fishman]

No.
No.

[a_canadian]

The more relevant problem is spacing of the ports relative to the chamber. Generally for keeping lower powered stuff subsonic it seems most go to around 4" for rimfire, sometimes closer. Some of the integrally suppressed Rugers get cut down to just 2" or 2.5", such that even high velocity stuff comes out below the speed of sound while keeping the OAL of the suppressed pistol reasonably short. Of course that's not porting... but a similar result can be had using a handful of 1/8" holes at 4" up the barrel. Further out and you're just releasing gases sideways, not reducing velocity much. In my own tests an inherently subsonic round actually comes out of the muzzle quite a bit faster at 11.5" than at 18", neither being ported, and significantly slower than rated on the box from a 4" barrel.