I got welcomed as the gunmaster even with no ranged techniques and not using a ranged power weapon. I did switch to one afterwards though, in case I get checked on it inside.
Edit: And after that first time it seems to have been fixed, so I can't get into the castle anymore.
Using a ranged deck I can get the result Al found about needing the right weapon. Neither automatic weapon or rifle weapon seems to qualify. And the power of a weapon doesn't seem to matter, since up to 10 ranged power doesn't do it.
I also tried using a close combat shotgun since I had a shotgun technique in my deck.
Using a sniper rifle seems to work, though! Not sure if it's because I have Longarm in my deck or not.