is the magazine designed to shoot both cartridges, 38spl and 357??
Its sized for the .357's. It may, or may not feed the .38's reliably. The slight difference in the OAL of the rounds is enough to to let the .38's jump rims, and the gun/mag can and will "rim lock" with them. The .357's feed and work fine.
Ive had the rim lock problem with the .38's. It doesnt happen "all' the time, but enough to be annoying. When it happens, you'll need to drop the mag and "unlock" the rounds in the mag.