All the guns I've seen pictures or descriptions of in this discussion are perfectly legal. Some are pre 1898, meaning that unless you live in a semi-fascist state like NJ, you can buy them without any paperwork. They are antiques.
A surgeon I know is quite wealthy, and a serious gun collector. He became part owner of an FFL gun dealership and also of a bail bondsman/private detective agency. Strictly a money investment in each. He takes no part in operations, but his name is on the paperwork.
He has more guns than the Danish army, and can carry concealed weapons in any state he chooses, perfectly legally.
The statistics regarding violent crimes and gun availability are very interesting because they demonstrate the lack of clear connections.
In some nations gun availability is very limited, and violent crime is extremely low. A few hundred kilometers down the road, guns are everywhere, purchase of pistols is easy, but violent crime is also very rare. Switzerland is a good example of this phenomenon.
In Japan guns are almost impossible to acquire, and all pistols are completely illegal for non-police/military personnel. Gun violence there is almost unknown.Their murder rate is fairly high, though.
They use knives.
And Sarin gas, as I recall.
also I read somewhere in my preparations to visit Australia that the same terrorist group once owned uranium bearing land in the outback, and they are thought to have been trying to develop a nuclear bomb, but eventually settled for the easier to deploy the poison gas.
The point is, you do not need a gun to kill people. Even lots of people.