So why is there a hand grenade in the Soprano’s secret storage space where Tony and Carm keep the guns? It showed up in the first or second episode of the season, and it was still there this week when Carm got the gun to put under her pillow (thinking about Tony after sleeping with the teacher).
Sure, we can all accept that Tony would be able to get such munitions, but why would he want one in his house?!? To hold off the FBI? And why would Carm want to keep it around after Tony left?
And are we to assume that the standard dramatic maxim (if you see a gun in the first act, it will go off by the third) will hold? Will the season or series end with a literal bang?
(Oh, and BTW, are we supposed to assume that Meadow and AJ have never cracked the secret of the hidden storage space, and Carm’s top secret hiding place for the key: in the flour canister on the kitchen counter???)
So, did nobody else notice this (you all think I’m nuts), or is keeping hand grenades at home more common than I thought?
I noticed the grenade as well. Most likely, it’s there because Tony thought it would be cool to have a live grenade in his personal arsenal. It doesn’t have to be particularly useful for anything. The chances of him actually needing it are pretty slim. The reason Carm keeps it around is because - well, how would she get rid of it? (That’s one garage sale I’d definitely be interested in visiting. 
)
On another level, I think it symbolizes the tension in the Soprano household - something has to blow eventually.
I don’t doubt that both Meadow and AJ know about how to access the arsenal. But neither of them has any need whatsoever for a gun or grenade. I can see maybe AJ showing off the arsenal to his friends when the parents aren’t around, but beyond that he’d just ignore it. Well, until the little baby is scared by a bear or something. 