Swifty52-
Your post includes several points that are worth comment.
Yep, 6mm lee navy is the parent case. Rumor has it from some old, old gun gurus that the original designer of the cartridge went to Winchester with his idea and they liked it.
In 1933, Grosvenor Wotkyns, a gunwriter and experimenter who had played a major part in the commercialization of the Hornet, took his version of the 22-250 to Western Cartridge Co., furnishing them with both his rifle and ammunition for testing. Western was able to obtain 4000 fps with the cartridge, and were so impressed that after some months they passed the rifle and cartridge to Winchester, an Olin/Western subsidiary since 1931. After more tests, Winchester decided to bring out the cartridge and rifles for it.
Problem was that they didn't own the rights to the case he designed it from.
Wotkyns's Swift was based on the necked-down 250 Savage case. My understanding was that Winchester management absolutely did not want to produce a cartridge based on a case from a competitor's cartridge. When asked by Winchester to recommend a substitute, Wotkyns suggested the Lee Navy case. Is there any published information about "the rights to the case" preventing adoption by Winchester?
So it seems they had the tooling and left over brass from the Lee Navy, when necked down to 22 caliber it had a little more case capacity.
This rumor about the reasons for the adoption of the Lee case has been circulating for a while. Because the 220 Swift case differs internally and externally from the Lee case, reworking leftover 6mm cases would be unlikely from an engineering view. Again, is there any reliable published foundation to this rumor?
You're correct that the Lee and Swift cases have somewhat greater capacity than the necked-down 250 Savage case.
It was a new cartridge for a new model rifle.
The 220 Swift was first offered for sale in the Model 54, which was not really new, having been on the market for about ten years. It was one of the first chamberings offered in the Model 70 a year after the cartridge was offered publically.
The semi rim is just part of the original Lee case, and why it wasn't changed is lost to time.
As pointed out elsewhere on this thread, the Swift case has a semi-rim designed to fit the standard 0.473" bolt face. The Lee case as loaded for the Navy rifle and the M1895 Colt-Browning machine gun was rimless. Here's a case diagram from ammoguide.com:
There exists a rare version of the Lee Navy case in a fully rimmed configuration. The Blake and perhaps other rifles were chambered for this cartridge about 1900. Phil Sharpe's
Rifle in America describes this.
--Bob
edited to add machine gun info