Possibly steel case. Steel case .22lr ammo does not like any sloppiness in a chamber. I had some old RWS that blew a rim or two in a sporting chamber. Best to pull the bullets, dump the propellant in the garden and hose it in as fertiliser then dump the cases in water or oil to deactivate the priming compound before tossing them. Alternatively, just chamber the empty cases and fire them off. No pressure equals no swelling.