You know you can get 45 cal air rifles...
Although I like the idea of a felony limiting someones ability to own a firearm, at least for the duration of their incarceration or parole and possibly a period after, it never seems to work well and I am sure any felon who is going to pursue a criminal lifestyle has no problems obtaining a firearm. Just another "sound good" firearms law.
http://www.pyramydair.com/site/manuals/bigbore/
I don't own one and have not looked into them much, but it might be worth looking in to.