I've used a variety of flasks over the years and my favorite is a horn with the springloaded valve at the tip. Those close off tightly and don't leak powder into your bag as most metal flasks will if carried in the bag or in a pocket. I much prefer the valve over a plain stopper since I don't have to pull a stopper and replace it every time and there is no danger the stopper may fall out and dump your powder.
It is probably good advise to not pour directly from the flask into the barrel since there is a very, very, tiny chance that a smoldering ember may remain in the bore to ignite the powder, although that is exactly the way they were meant to be used and were used for hundreds of years. When loading a revolver however I will never use any method other then pouring from a spout on the flask, there are just too many things to juggle when loading a revolver without adding a separate powder measure. That is where I use the metal flasks with a variety of spouts, some of which I have reamed with a tapered pin reamer to throw the exact charge preferred by a certain revolver. You should also know that most of those flask spouts do not throw the exact charge they are marked for.
