For safety, Always use a separate powder measure to measure the powder before it goes near any flame, ember, or other source of ignition. If you buy the small, plastic " speed loader", that carry one load of powder each, you measure and fill those speed loaders at home, and away from your gun barrel. The ones I have seen have tabs so that you can push the top off with your thumb while you hold the speed loader in the same hand. Opening the top does not require the use of two hands.
The safety concern should always be two fold: ONE, that their might be an ember still burning at the bottom of your barrel when you pour the next charge of powder down the barrel, igniting it and sending flames and unburned powder out the muzzle to burn or pepper your hands, and face. ERGO: Always load the barrel with it pointed away from your face, and keep your hands away from the muzzle when you pour in the powder or or seat the ball, and do NOT wrap your thumb around aorund the ramrod when you are loading the ball down the barrel, in case the powder ignites and send the ball and ramrod back out the barrel. Use a hand over hand " Monkey Grip " with the hands no more than 8 inches apart on the rod to run the ball or conical bullet down the barrel. ( The Monkey Grip means you just curl the four fingers around the rod and hold the rod with friction of those fingers. If the rod were to be shot out of your " grip ", the human reaction will be to open the fingers up, rather than to grip tighter, and that opening of the fingers will save them from anything more than a severe burn. At least all the fingers will still be attached. If you were to have your thumb wrapped around the rod when it went off, you could lose the thumb! )
SECOND: Always isolate your powder source( Flask, horn, can) from the muzzle of the gun. If a spark ignites a powder charge, the flame will shoot out the muzzle before you can possibly react. If you are pouring powder down the barrel from a flask or horn, that BOMB in your hand will be ignited and explode in your face. It will certainly do you much harm if not kill you, and it can hurt anyone standing within 10 feet of you, and more.
I do not recommend using horns and flasks with the powder measure attached to them, or incorporated into the horn or flask. The so-called " English-style " measure, which has a pivot gate arrangment to open the powder source and measure a given amoount of powder between the first and second gates, then release that measured amount when the pivot is allowed to return, closing off the powder source, works poorly at best. Both gates get coated rather quickly with powder residue, which is enough itself to allow ignition. The powder measured is rarely consistent from shot to shot. And the gate is operated by a coil spring, and rarely is that spring strong enough to close the gate, or cut-off, by itself, against the pressure of the weight of all the powder in the flask or horn bearing down on it. Remember that you use gravity to move the powder from the source to the measure, which means the horn is upside down, and above the muzzle of the barrel if you ignore these safety warnings, and use it to pour a measure of powder down the barrel. That puts the flask or horn at face level, with the opening to the source right where flame from ignited powder will efficiently go right up into the source to ignite 1/2 to 1 pound of Black Powder. You might was well pull the pin on a Hand Grenade and hold it in front of your face. The damage will be the same.
ALWAYS USE A SEPARATE POWDER MEASURE TO PUT A SINGLE CHARGE OF POWDER INTO YOUR BARREL. KEEP YOUR POWDER HORN( OR FLASK) CLOSED AND AWAY FROM YOUR GUN DURING LOADING OPERATIONS.)
In my gun club, when we have a campfire going, or even have a wood stove working in our small shooting shack, we don't even allow members to approach the fires carrying their powder horns or flasks. Guns, horns( flask) and possible bags are removed and left away from the fires. ( NO LOADED GUNS IN CAMP.) And we consider a gun loaded if it still has a charge in the barrel. Members clear their guns on the range before coming to camp.
Hope that helps you understand how this all works together to make you safe, and safe to be around other people.
Go ahead and either make up single load containers, or buy the " speedloaders" on the market for this purpose. They make going into the field with a rifle much simpler, particularly when its rainy or snowing outside.
The only thing I feel comfortable about measuring out of a flask with one of those attachments is my corn meal that I use a filler on top of the powder charge, and behind my ball or bullet. My flask now has a 20 grain volume tube screwed into it. To use it I put my thumb over the end of the tube, upend the flask, open the gate to the flask, fill the tube, close the gate, and then pour the corn meal down the barrel. Done.