Been shooting Blackpowder firearms since 1973 and have never, ever, dropped a charge down the bore of a rifle directly from a flask or horn as I was taught this is a BIG no-no I'm a firm believer that one should always pour to a measure and then to the bore. I carry pre-measured charges when hunting. However... I do, as Cayugad mentions, use a flask to charge the cylinders on my BP revolver.

BPS

19th century powder flask (maybe earlier) had adjustable spouts as many of you may have some, these typically were graduated in drams or grains.
It's against the rules at the NMLRA to pour directly from a flask or can into the barrel. Although quite rare, 'dieseling' seems to occur on the shotgun lines more than I've ever heard on the rifle lines. Blowing down the bore is also not allowed.

People will do what they want to do no matter what. I was hunting small game with my dad and brother one day back in the 70's. We ran into another hunter and we stopped to pass the time of day, As he was talking to us he had the barrel of his shotgun resting on the toe of his boot, as trap shooters are want to do. I asked him if his gun was loaded and he said yes, I then said he may want to either unload it or remove the barrel from his toes. He laughed and said it isn't a problem, I shot off two toes a couple of years ago resting my gun the same way, the part of the boot the gun is on is empty. We had a good laughed about it at home after we cleaned our game over a beer, but it really was more sad than funny.

The way I have been doing it is using a flask that the powder measure "Plugs into". Use the valve on the flask to fill an adjustable powder measure, then load the gun from the measure. I have a couple photos but haven't figured out how to post them.