It isn't a question of how much or how messed up they would get, although .42 oz of grass should get them higher than they want in one sitting unless it is real crap. The question comes down to this: our culture has problems with people in posession of controlled substances and firearms at the same time. Heck, we don't even like drunks running around with a loaded gun.
If you think the law is in error, change the law, or defy it at your own risk.
The issue here isn't really a Second amendment issue, it is a firearms/drug posession issue. If the drug had been LSD or ecstacy, would you still feel the same?