Yup, because if you ban them then every criminal will just hand them in and will never be able to obtain another one on the black market.
I personally don't think that that guns should be banned, and people should be allowed to own guns to protect themselves and their families.
That being said, your point about criminals not handing in guns, and criminal access to guns on the black market, is not entirely relevant. Here in the UK, any criminal/gangster that wants to obtain a firearm, could do so if they wanted it bad enough. I, on the other hand wouldn't have a clue as to where to start looking for one. In the US, I would Google the closest gun shop, walk in with my drivers licence (in applicable states) and walk out 5 minutes later with whatever gun I fancied.
From my understanding, most of the major massacres that took place in recent history (Columbine, Virginia Tech etc.) were perpetrated by individuals with no criminal links/history. My guess is, that in a place like the UK, these individuals would not have been able to gain access to these weapons, thus preventing them from carrying out these atrocities.
My point is that, yes, criminals would still have access to guns, but we should be more afraid of your "average Joe" that goes crazy and takes an AK-47 to a building full of children.