That is indeed, an odd one. Never heard of a Savage 340 in .225 Win. .30-30 and .222 Rem were the common calibers. I think there might have been a .223, for a while, but I'm not sure.
The .225 Winchester is one of those rounds that probably never should have been made. It got off to a screeching halt when introduced, and while they did sell a few and some other makers did make a few, it was only a few, and Remington standardizing the wildcat .22-250 a year later, basically ended the little popularity the .225 had.
The .225 is a rimmed case. Often said to have been developed from the .220 Swift (which was a development of the 6mm Lee Navy case), I just checked some drawings, and while the .225 has the same rim size as the .220 Swift (.473" which means it fits standard bolt faces) the case body of the .225 is about .02" smaller in diameter than the Swift, and its also shorter.
The .225 Win will do 55gr slugs in the 3600fps range, but so will the .22-250, and its rimless. The .225 simply wasn't "modern" due to the rimmed case, and faded away. Ammo and brass is where you can find it, for what ever they are asking. Good Luck.
Oh, do take extreme care with that Savage 340 magazine. They are nearly
"unobtainium" in the standard calibers, and I would be amazed if you could ever find one for .225 Win. The magazine might be worth more than the entire rifle!!! There's probably at least a few "single shot" Savage 340s out there, because the mag was lost or damaged.
The .225 has decent performance, you can match most .22-250 loads, but the cases are rare, out of production, and cannot be easily formed from anything else you can easily get..
I just checked some more drawings, you might be able to make a case that would work from .30-30 brass, but you'd have to lathe turn the .30-30 rim from .506" down to .473" and do some other work. And the .30-30 specs 0.415" at the case head, the .225 is 0.422" so cases made from .30-30 would still be slightly undersize at the case head. This would work, once, anyway
but I don't know for how long, or how well with full power loads.
Good Luck with yours!!