[taliv]

so i'm getting geared up to reload 220-swift and looking back over the history of the cartridge and the 6mm lee navy etc.

the thing i can't figure out is the rationale behind what i'm assuming is a compromise between rimmed and not-rimmed: semi-rimmed

anybody have any insight on why that design was chosen?

[JesseL]

My wild-a**-guess is that somebody was concerned that a rimless design wouldn't have enough room for the extractor to grip, so they went semi-rimmed to be safe. That or they wanted a round that would feed from a box magazine but would still headspace on the rim.

[MachIVshooter]

Semi-rimmed cases are similar in functional theory to belted cases, being that a certain cartridge often did not have enough shoulder to properly headspace. One cannot be entirely sure why this feature was retained on the .220 swift, a cartridge that is almost overbore. There is certainly plenty of shoulder to headspace.

[Jim Watson]

The .220 Swift (and the .225 Winchester) are semi-rimmed so as to bring their rim diameter up to .473" for use with standard bolt faces and extractors.
Nothing to do with headspacing or feeding in a bolt action rifle. Just that their parent cartridges were of smaller casehead diameter than .30-06 etc. and Winchester did not want to have to make a different bolt.

[JesseL]

So the 6mm Lee-Navy was rimless, and Winchester added a rim?

[mete]

Yes , I wonder if it had to do with the rifle used.BTW, the 6mm Navy actually saw combat use in the Boxer Rebellion in China .

[MachIVshooter]

The .220 Swift (and the .225 Winchester) are semi-rimmed

I don't have my copy of COTW in front of me, but IIRC the .225 Win. is rimmed, not semi-rimmed. I'll look it up later, though.

[Peter M. Eick]

In the may -june 1972 issue of Handloader page 48, Ken Waters says that Winchester calls it semi-rimmed. The 225 is a derivation of the 219 Zipper by the way.

I just happen to have that issue open in the background while I was waiting on the computer to download pages. Good article on the 225. Waters actually states on pg 49 that the 225 is "one of the most accurate of present-day factory cartridges, possessing tremendous handloading potential as well."

[Jim Watson]

CotW calls the .225 Win. "rimmed." I said semi-rimmed because the flange (in British terminology) plays no part other than to make the cartridge fit an '06 breechface. It headspaces on its shoulder in its parent rifle, the post-'64 M70. There was a brief spate of enthusiasm for converting various single shots to .225 Win. so as to not have to build a rimless extractor.