Not sure what you mean...I believe Afghanistan and Iraq currently require machine guns...as well as Georgia...
Of course not!
It depends on what you mean. In most U.S. states you do not need a permit to own regular handguns, shotguns, rifles, or black powder guns. In fact, you actually don't need a permit to own any firearms, but some firearms (such as machine guns and short barrel guns) require registration.
Yes
Probably about the same as any other country's machine guns in any wars they've won. We weren't the first to have them - by the time we employed them, our enemies did, as well.
There is not any specific prohibition against the use of fully automatic weapons ("machine guns") in warfare in the Geneva Convention treaties or any others with which I am familiar. Countries which have laws against automatic weapons generally write them in such a way that they specifically do not apply to properly authorized agents of government (such as soldiers) using them in the course of their duties.
Yes, but not any more than any other gun.
Cost.
Machine Guns, like any other gun (with the possible exception of an air soft gun) could seriously harm, and even kill you if at point blank range. Any gun could kill you, so, yes. a machine gun is powerful.
Well there is assault rifles but not any sub machine guns that I know of.
USYes. Homemade guns are legal. You may make a gun for your own use provided it is not of a type specially regulated by the Bureau of Alcohol, Tobacco and Firearms without any license. Guns made for sale to others, machine guns, Destructive Devices, short barrel rifles and short barrel shotguns would require either a license or a tax stamp from ATF.
Pretty much any nation which ever had a military since the 20th Century.
because we have machine guns and tanks and naphalm