Not sure you can close a gap of that size by wetting. I would plane and glue a piece inside the barrel channel then re-inlet the barrel.
I doubt the gap would completely close, and upon drying would open again, but not quite as much. Rich is correct, a wood patch is what is called for. Done carefully it will not be unsightly, do stain the parts (edges) that are to be inlet before gluing in place. In the following photo's you will see a repair I did along the barrel channel, your techniques will be similar but near the lock inlet, go slow, be sure. Always be test fitting as you go and do a clamp test before actually gluing in your piece.
The technique is to first fabricate a bit of wood to be inlet, then to chisel out the area you intend to glue it into.
Now is when you do a test clamping to work out how you'll proceed once you stain and add glue - to both pieces.
The repair (of a worm hole in this case) was successful. Once the stock was shaped you could barely tell it was repaired. Good luck Bill, and welcome to ALR.
dave