I know there are varying opinions on this, but when working on the two barrel-cracked Murrays I've had (one came that way; the other, like yours, cracked in the winter) I haven't worried about "closing" the cracks so much as FILLING them, and they've worked fine for several years now. I'm kind of going on the theory of "it cracked because of stress so why re-stress it or return it to the shape it was trying to get out of (kind of like too-tight pants
)?"
Anyway, the most important (and hardest to find) component in my repairs has been patience. Here's what I did:
I put masking tape near the edges of the crack to minimize drips, applied some super-runny superglue (Loctite is the brand I used) with a wooden toothpick (you'll need several as the glue dries fast) and let the glue set for several hours, even overnight. Then I rubbed 000 superfine steel wool around the crack, GOING WITH THE GRAIN to clean up any big drips (this is important to do at each phase or you end up with small, sort of carbunculous-looking blobs that just don't want to go away without a major fight
).
THEN, I repeated the process several times (I think it was five times on one flute) until the fill was level with the flute surface. I let it set again and then removed the mask and carefully rubbed the glue repair and the crack edges with the steel wool to sort of "smooth the transition."
In each case, the gluing and setting process was over several days; I wanted the glue to cure as well as possible.
In the several years since their repairs, the flutes have been fine and in the case of the blackwood flute (the one where I figured out about the steel wool between each gluing), the crack is all but invisible in most places.
The one thing I'm still trying to figure out is the best way to fill the crack from the inside. On my boxwood Murray's cracked foot (talk about a hardworking flute; it came with plenty of battle scars) I used a long wooden skewer to kind of dab the glue into the crack and then attached some steel wool to the tip of the skewer to polish the bead from the inside of the bore. It worked and was fairly smooth (I've since had John Gallagher do a professional repair on it with epoxy and boxwood shavings because it was a big split so it's much more attractive now), but I know it could have been better -- and it was a major pain to do.
Anyway, they're both great flutes and like yours, working flutes so I figure the cracks are kind of like service medals.
P.S. Acetone on a swab is good for emergency superglue cleanup.
Deja Fu: The sense that somewhere, somehow, you've been kicked in the head exactly like this before.
. The crack runs from the edge of the barrel (nearest the headjoint) down to the raised trademark curve Sam puts on his barrels. 
