In my opinion, this is a big problem for folks who wish to ban guns of any type, or any accesory.
The Illicit Arms Trade
This link shows black markets have already been established to meet the demand by people who either can't get firearms, or people who want guns, magazines, suppressors, ect. which have already been banned. If you banned Semi-auto rifles, and/or high-cap mags what makes you think the black markets wouldn't simply step up to meet the additional demand? If you acknowledge that they would how would you solve this problem?
True, it does somewhat parallel drugs in that manner, however there are some differences which would make a ban on guns much easier and more effective than banning drugs. I am just discussing this as a discussion, not condoning before the crazies start whining.
First off the nature of drugs is diverse and many of them simply pop out of the ground. If a country were to ban guns it would be able to also ban internal manufacture of the weapons and ammo. Unlike the drug war, you would then only need to worry about imports.
Guns are easier to scan for and detect in packages. They have a specific design and shape and are often composed of metal or materials that would show up on an X-ray scan or on a metal detector. Not to mention they are hard materials. This means they are not easily conformed to odd spaces like drugs can be. So you really are at a much bigger advantage in keeping imports out.
makeshift guns do not have the potency of regular guns. Yes, there are things like zip guns and things where you could fire a bullet, but these things do not have the accuracy, firing rate, and utility for shooting that a gun has. This renders most jury rigged stuff far less dangerous.
Due to these natures it would be easier to strip an area of guns and to keep guns out because manufacture inside that area would be hard. Slowly you would remove the guns in circulation and lower the number available. With a good crack down this would drive gun owners further underground where they would resort to other weapons saving the gun for extreme situations because they would not want to blow their power on something petty.
Now it is true that in the US there are more ways to import due to less secure borders than say the UK. The UK has an advantage that you cannot walk guns in nor bring them by car. The US has large borders and a big area. So that does mean there is a larger area to bring guns in and perhaps also a large area to manufacture guns in the US if you could get the money up for the equipment. Of course the crackdown would drive prices sky high on guns being unlike drugs they do not just pop up from nowhere and have a much more resource heavy creation process. If there was a crackdown and a ban there would rise a black market for guns. It is already here so you are not going to stop it. What a full ban would do is to drastically lower the number of guns in circulation and allow for the prosecution of those who would own or make one. Not everyone chooses to have a gun. This is why we see some countries having success with gun banning. I think a real gun ban would be a lot more effective than a drug ban simply because guns are harder to hide than drugs and harder to make. It would be hard at first to round up the guns in the US given the number of them out there, but as long as you were destroying them you would probably be able to seize more than were imported or manufactured if you made a good effort to watch imports and internal manufacturing.
My personal opinion is it costs too much to do all of that. A war on guns would be like a war on drugs in that we would criminalize a lot of people who did nothing wrong aside from possession or safe use of a gun. This is why I would prefer a more heavy licensing structure with taxes to help make things run and cover any damage. It is the same thing i support with drugs. You can have them, but you have to do the right thing.