Check out http://en.wikipedia.org/wiki/Taxing_and_Spending_Clause for some interesting information on this in layman's terms. Looks like there wasn't an agreement on the reach of this clause by the founding fathers.

But under Supreme Court case law, Congress has pretty broad discretion in using its spending power.

As to your question:

No, they can't do anything under the general welfare clause. The Federal Government is bound by the enumerated powers in the Constitution. So while say forcing the states to do something might be beyond its power, it can entice the states to fall in line using the power of the purse strings. Of course there are some restrictions. See South Dakota v. Dole.

Check out http://en.wikipedia.org/wiki/Lists_of_countries_by_gun_ownership. Per 100 people, the United States has 90 guns. Canada has 31.5. All I can say to that is, "weak." France and Finland are beating you (which I wouldn't have expected). Switzerland up there doesn't surprise me though.

Not sure why the U.S. is so ridiculously high. I guess a better statistic would be the percentage of people who own guns. I'm pretty sure a lot of people who buys guns buy a lot of them.

Now check out fire-arm related deaths at http://en.wikipedia.org/wiki/List_of_countries_by_firearm-related_death_rate. Man, the U.S. is awesome. I've never felt more proud to be an American.