I've had good results with the beer gun. Here's my procedure:
1 day prior to bottling, increase the PSI to an additional 2 PSI - I carbonate at around 10PSI, so I increase to 12 PSI. Put the sanitized bottles, covered, into the freezer.
Day of bottling: Vent the keg, then reduce PSI to about 8 PSI. This is the pressure I use for the beer gun.
I set the CO2 pressure (for the venting part of the beer gun) to 6-10 PSI (I have 2 CO2 tanks) but you could also leave it at 8 PSI as well. The venting CO2 pressure isn't too critical.
Vent the bottle with CO2, for about 3 seconds. The sound changes when the bottle is vented.
Fill with beer until a small amount of beer comes out (may be 1/8oz?). Remove beer gun, and cap immediately.
I hope this helps a bit.