I like to have a separate bag and horn for each rifle, or at least calibre, but a separate bag is the only important thing. I can use my 3Fg horn(s) for a variety of rifles, and similarly with the 2Fg horn(s). I also have several priming horns that I carry on cords around m' neck.
So here are a few of my favourites...
The bag above I purchased from a member of this forum several years ago. It is my .50 cal bag. It has a lined flap, a hanging pouch for balls on the inside back, two compartments and a pouch under the flap. I like these features as it organizes my stuff, and I keep the same theme with most of my bags.
These three images are of the same bag. It is my Hawken bag. It and several others of my bags are made from some great leather I bought from one of our members several years ago, and I still have some left over. It too has inside and outside pouches and compartments. It is accompanied by a large horn that will take a full pound of powder.
These images are of my .40 cal bag. I have three rifles in that calibre, so it gets a lot of use, as you can see. It is made of brain tanned moose hide, lined with canvas, and edged with brain tanned deer skin.
And last is my smoothbore bag. It services my Chambers' Penn fowler 20 gauge, mostly with patched round ball. It's made of canvas lined with denim patch material, per TC Albert's great article in Muzzlelaoder mag a couple years ago. I made my haversack using the same design, but bigger. It is a double bag.