Honestly, I think powder horns are a better choice. The horn doesn’t “sweat” with rapid changes in temperature, the shape carries well on your side, and a properly made horn is airtight, which protects the powder from humidity. With that said, the charger top of a traditional flask sure is a convenience. I have several flasks, all modern-made but traditional styles, and use them frequently on trips to the range. Before comments are made about safety, I will say that I dump the charge from the flask into an oversized (100 grain) powder measure, and then pour from that into the barrel. We don’t pour directly from the flask into the barrel these days.
However, as one who has handled a few antique flasks, I would like to point out that they are inherently fragile. They will handle use in the field with reasonable care and last for generations, but they are simply not built to withstand being sat or stepped on, or trampled by a horse, or slammed in a car door. How on earth did a flask get smashed in a bag?
Respectfully, maybe a change in how one‘s gear is managed and handled might be preferable to getting an armored flask!
Best regards,
Notchy Bob