I think it's perfectly reasonable to keep someone who's been shown to be criminally irresponsible with firearms from having access to them. Ever. (But then I am about as anti-gun as they come, make of that what you will.)
The real cause for concern here is that drug offences that shouldn't be offences in the first place are treated as felonies, so that anyone with a personal interest in changing the drug laws is effectively silenced. And of course, anyone who's read a bit of history knows that drug laws (those of the US in particular) are not much about public health and very much about identifying and neutralising non-conformists.
And let us not forget that marijuana was outlawed specifically because it was seen as a "Negro" drug...
BTW, once you've done your time, you can apply for re-enfranchisement. I'm not saying it's always granted, but it does happen.