Salopian has more experience than me as a loader, but I'll throw in my two pennorth anyway.
Bags of this type are common on driven shoots, and are invariably carried by a loader. The Gun may be shooting with a pair or a single gun. Bear in mind that a drive can last for over half an hour so the action isn't steady and continuous, it's a mixture of slow periods and very intense interludes so speedy gun handling and loading are of great importance. Keepers and estate managers are really pleased when they present a couple of hundred high birds in a flush and some clown is fumbling about with his gun, and failing to fire.
When big numbers are expected the loader will have two bags with him, one for ready use, one for reserve. 30 cartridges aren't anywhere near enough for one drive on a big day not even near it; I've seen eight Guns take 1000 shots on some drives often enough.
Eug
