Not sure if this is proper way, but it is how I have done it three or four times on various Hawken style rifles:
With the barrel unloaded, dismounted & upsidedown... tap the center of the barrel wedge (key) lug with an object like an old dull chisel or screwdriver. The idea is to peen down the opening so the barrel wedge (key) contacts the lug with more tension.
Do a tiny bit at a time retrying barrel wedge fit in assembled gun until you get adequate tension.
The downside of this method is a welded on barrel wedge lug may detach from barrel. Has not happened in my doing it.
If the barrel wedge lug is screwed on seems you could just file off some material to reduce the opening.
Just my 2 cents.
Curious if there is a much better way?