I found (and didn't buy) a magazine at a flea market yesterday and now I'm wondering if I should have pounced. The only markings are "284 Win." The bottom was very round. The seller thinks it was for a Ruger, because he used to have a bunch of Ruger parts.
What model has a round bottom detatchable magazine and was chambered in 284 Winchester?

The Winchester 100 self-loader or 88 lever-action come to mind. The Browning BLR also came in 284.

I'd bet that was for a Savage 99.

I would concur with Larry, who probably got that opinion from looking at my 99C. I always wanted to build a 35-284 on it, both traded it in a moment of weakness.