brickeye,
You are purging the interior of the barrel.-YES.
It does not matter since you are welding on the exterior of the barrel.-
You haven't a CLUE !
You seem tyo have no idea what you are doing.
If you wanted to attempt to weld a barrel foot on a 1911 barrel the welding is occurring on the OUTSIDE of the barrel.
The purge would be to prevent oxidation from heat on the INSIDE of the chamber.
The purging gas will not be anywhere near the welding.