The Saiga magazine will accept cartridges up to 2.75" long.
It will, of course, accept rimmed cartridges.
Other magazines could be adapted.
The bolt is flat, with no recess. Only the extractor hooks have to be adjusted to accept any rim size.
The firing pin is sprung. Stoutly, too; stouter than the Polytech spring I bought for my Beowulf build.