I’ve heard more than a few gun control proponents say privately that if people didn’t own guns, there wouldn’t be any way to steal them. Publically, they won’t say this, because Handgun Control Inc. The Brady Campaign supposedly isn’t in favor of banning handguns any longer, at least publically.
One of these days I’m going to write a post on substitution.  There’s some interesting statics that I’ve run across that I want to share, which basically show that if you close down one means of criminal access to firearms, it boosts other means. Create background checks at point-of-sale, you get a straw purchase problem.  Shut down straw purchases, you get a theft problem.  Shut down theft, you get more organized smuggling. There will always be someone to fill the demand, which is obvious to us.  But the anti-gun folks are always going to push to close that one last “loophole” until we can’t own anything anymore.
Aren’t straw purchases almost impossible to catch? Unless the police perform a randomized house search to check a person’s guns (like some countries do) it is almost impossible to tell if that gun was passed on to a criminal or not (until it is recovered at a crime scene).
Criminals sometimes make their own firearms, such such zip guns, which can look like anything (lighters, cell phones, flashlights, etc.). An enterprising criminal group could probably manufacturer more advanced weapons.
Yes, they could. Straw purchase is tough to prosecute, but not impossible.