If a legislature chose to pass laws to make people convicted of certain crimes be lifetime slaves of the state even after they are paroled, it would be within its authority to do so. Congress, however, has no authority to mandate such punishment for state crimes.
As I said before, though, I find it interesting that HCI et al. don't mention the only way in which a prohibition against felons bearing arms might be constitutional. Do you agree that HCI et al. have a real agenda they don't want people to know about?
No, the "slavery" is while they are in prison/probation/parole. When they are released from said, they are no longer "slaves".