I asked my dad about pouring from the flask last night when he called. He's a regular fount of information about amazing stuff.
His grandfather (my great-grandfather) had an 1860 Colt that he shot every now and then. Dad remembers that he would charge it from the flask, filling it up just a bit short of full, then put the ball over that. No wad, no grease, just powder and a ball. Why did he do it that way? Because
his dad (my great-great-granddad) did it that way.
I don't think that this was a problem back then because nobody shot round after round after round from their guns. They loaded 'em up for a purpose - in great-grandpa's case, it was for hunting - and were very frugal with both powder and lead. "Target shooting" was hunting and you didn't take many shots. Thus, that revolver was always loaded cold. Oh, and his "flask" was a small tobacco tin.
I sure do wish that they'd hung onto that revolver (and the many, many other guns that great-grandpa had). It ended up in the trash, along with a lot of other guns when he moved from North Dakota to Idaho. Back in the '30s, things were a little different
. At least we've still got the 1862 Springfield.