Black Market
If someone is a "violent felon" and you send them to jail, then clearly you will keep them there until they are no longer a threat to society.
I mean, the parole board would not let them out otherwise, right?
Now, you have a kind of "limbo" status while you're on parole. You're still in prison but not really. During this period, you don't get to own weapons.
If you successfully complete your parole, you've demonstrated you can re-integrate into society, right? If you screw that up, you go back behind bars.
You can't control the commodity itself (in this case, guns), and the attempt only creates a black market.
If the objective of your "penal" system is rehabilitation, then do it for real, turn them loose, and give them back their rights. If a guy can't be rehabilitated, then you don't let him loose, since he'll just do something violent again. And he'll get his equipment on the black market.
If rehabilitation is impossible, then we quit kidding ourselves and keep them locked up. Tighten up the sentences until actual deterrent effect kicks in. Death penalty as needed.
What you DON'T do is house them for a while, decide you have no room, let them go, and then have a stupid "rule" (that they will ignore) about what they're allowed to own. Once you let them out, you don't control them any more.
And the alternative SUPREME STUPIDITY of trying to control what EVERYONE may own, so that people you can't fix and won't restrain "can't get them" is among the more mind-boggling idiocies of our time.
If they are trustworthy enough to let out, then leave them the hell alone.
Otherwise don't let them out.