They seem to be enforced in the many countries with a fairly high degree of success. Most criminals in the UK don't come armed with guns. They could probably get one if they put in some real effort and cash, but most don't bother. Aside from anything else there is little point, since most of the non-criminal population isn't armed either. A knife works almost as well, or better still just going undetected.
Think about it for a moment. If controls were completely unenforceable in the US criminals would be able to import more powerful weapons. Grenades, C4, RPGs etc. Those things exist in the US, but are not widely stolen or imported by criminals for some reason.
The reason gun control is impossible in the US is that it's an arms race. Law abiding people won't give up their guns because criminals have them, and criminals want to carry guns because law abiding people have them. The limiting factor is the government, which tries to disarm criminals and limit the guns law abiding citizens can own.