On metallic cartridge guns and breechloaders, the receivers were recycled into other firearms or calibers. Unwanted barrels are leftovers from that process.
Gary is quite correct, with many of them ending up on the Plains as Buffalo Rifles!
J.
I am no expert on the historical aspect of this, but I'll add my modern experience. Mind you, this was my first and last experience of this type, and I was (and am) a rank newbie in this area.
In the early 90's I came across a Remington Rolling Block #1 at a garage sale. It had a burst .45-70 barrel (about 12" from the muzzle) that her deceased husband had owned. I relieved her of it for $100.
Then my imagination went wild.
I completely disassembled it and the receiver was in very good shape, and only had to replace a couple parts which I obtained from Numrich. The case colors were faded, and a friend and I managed to heat the barrel a bit and remove it.
I ordered a 34" barrel chambered in .45-120 from some company (?) and we installed it to the frame, and magically the top barrel flat came up perfectly horizontal with no fitting.
I then ordered 100 new .45-120 brass casings from SMI (?) from Montana and loaded 20 0f them using RCBS .45-70 dies (adjusted high) on an RCBS single-stage press using 405 gr leadSpeer bullets and IMR 4895 powder (smokeless).
We tied the re-assembled gun to a tree and used a 50' string to pull the trigger. It didn't come apart so we did it for several more rounds like that and everything held together.
I then ordered some fancy walnut wood (butt and fore-end) from E.C. Bishop in Warsaw MO (they were cheaper than Reinhart Fajen in the same town) with crescent buttplate. I spent nearly 3 months using Linspeed oil on the wood.
Added a tangent caliper aperture rear sight and a globe front sight w/level.
When it was all finished, it shot well, but it sure was a heavy rifle. My, my, I had my pseudo Creedmoor!
After several years I got burnt out on it and told a co-worker about it. He took one look at it and offered me $2000.
I took the offer and broke even, without adding in the sweat equity, which was the learning experience. He died a few years later, his wife did not like guns, we moved out of state, and I have no idea what happened to it.
I got lucky and I won't attempt anything like it again!
To the OP: that's where some frames end up.
Jim