.22 LR detonating inside a tumbler, however remote that possibility is, would not likely pose much of a threat. I wouldn't want to test it out, but my guess is that without adequate containment to build pressure and nothing to "push-off" against, the bullet would not penetrate a plastic tumbler. Even if it did, there wouldn't be enough residual energy to be lethal. Again, that's just an educated guess; not an excuse to clean .22 LRs in a brass tumbler.
I only load .44, .45, and 7mm Mag. so I can mix and clean without issue.
