Well fellers, this is the first day I've gotten to see this thread. This ain't necessarily a reply to Bartek's post, I'm just putting in my 2 pence here at this point.
Over a period of more than 30 years of shooting ML's I've been guilty of blowing down the barrel from time to time. I still do it after washing out the barrel after I'm done for the day, basically to blow out as much moisture lingering around the cone of the touchhole or the bolster in my percussion guns that my dry patch can't reach. But one day several years ago while shooting I got to talking with somebody and forgot where I was at. I was sure that I had fired and went over and blew down the barrel. It was plugged. I blew harder, still nothing came out. Then it hit me, maybe I didn't shoot. I took the gun to the line, primed, and touched 'er off. "BANG!"
Now, when I blew into it, it wasn't primed. And obviously there wasn't a hot ember in it or it would have cooked off when I dumped the powder in it. But it still scared the manure out of me. It broke me from eating gun barrels on the line. Like I said, I still will blow after washing the barrel. I know it's unloaded then. But when you're at the range and talking, etc., well...
If you're hunting and fire a shot, you can be pretty sure it's unloaded, but, I can tell you, it can be a hard habit to break and you will find yourself doing it at the range and at the very least you may find yourself kicked off of it. At the worst...
As for your breath softening the fouling, that's a joke. If you have breath that wet, you're going to have to run a dry patch down anyway to wipe your slobber out. If not, you're going to wet your powder. Blowing into a fresh fired bore can also loosen a flake of unburned powder or residue and lodge it into the backside of a coned touchhole requiring you to run a wire into it anyway.
If you insist on blowing anyway, try this: Keep your face away from the muzzle and blow across it like we used to blow across pop bottles to make sounds when we were kids. Enough air will go into the barrel to expel the smoke still in there.
As for a cannon discharging when ramming the load; If properly swabbed and wormed, there shouldn't be anything in there. But, over the centuries, cannoneers knew they couldn't be sure, so the gunner wore a leather thumbstall and kept his thumb over the vent while it was cleaned and rammed to make sure fresh oxygen didn't get in. I have a book written by a cannoneer who describes one such man who was mortally wounded while thumbing the vent and although he was dying and sinking to the ground, he kept the vent covered while his comrade rammed home a fresh charge. The writer made the point that the man knew his duty was to protect that comrade from a pre-mature discharge. Hot embers weren't the only danger to those men. They often fired so fast that the barrels grew dangerously hot. And flannel powder bags are not the best insulators. I have also read of modern artillery cooking off due to overheated barrels. The limits on how often reenactor artillery crews can fire has greatly reduced the possibility of cook-offs.
This has been an interesting thread. Lot's of pros and cons with logic to both. But as for me, with the exception of maybe blowing water out I think I'll leave the barrel blowing to the Hollywood cowboys.
) were old timers about using their guns. One example: it was obligatory for every one musketer to take 8 balls (not 7 or 9) in the mouth and keep them in the mouth direct befor the battle. 
