Firearms are easy enough to make. One of the reasons bans are pointless.

You could even whittle one from a block of wood if you wanted to.

If you're in a situation where you feel the need to rely on an "improvised" weapon the law of the land is already gone.

I wouldnt count things like bows as improvised weapons. They are weapons. They arent firearms but they are certainly weapons.

Keep in mind that an improvised weapon only needs to last long enough and function well enough to enable you to acquire a true weapon.

Pipe and hammer, wood and nail, stone and spring, an infinite number of junk will get you a functioning firearm. Suppressor too. All you really need to know is how a firearm works. They practically build themselves.

In WWII the US made these craptastic .45 pistols that were basically just folded metal and a pipe to be dropped to French resistance fighters. They were single shot pistols intended to just last long enough to kill an enemy and acquire a more substantial firearm.

Theoretically you can melt down a bunch of aluminum pop cans and file, drill and grind out a perfectly serviceable weapon with nothing but hand tools.

It all depends on how much time and effort you're willing to dedicate to your final product.