Video images showed the assailant fired at Abe with a device that had a pistol grip and what appeared to be two pipes covered in black electrical tape. Police arrested a 41-year-old man at the scene and said he had admitted shooting Abe; the suspect was later identified as Tetsuya Yamagami.
"Anyone with a basic understanding of how guns work could have made it with minimum knowledge," said firearms commentator Tetsuya Tsuda, adding that it may not even have taken half a day to manufacture the weapon used in the attack.
Japanese media said on Saturday the suspect had told investigators he had searched online for instructions how to make firearms, and ordered parts and gunpowder on the internet as well.
The gun measured 40 by 20 centimetres (15.7 by 7.9 inches), and was made of materials such as metal and wood, officials from the Nara prefectoral police told reporters on Friday.
Police did not rule out the possibility that the bullets were also made by hand but said they were still investigating.
Investigators seized what appeared to be five hand-made guns from Yamagami's house, the Mainichi newspaper reported on Saturday.
Simple to make
"Crude, yet lethal, craft-produced (...) firearms such as this are simple to make," said N. R. Jenzen-Jones, an arms and munitions intelligence specialist of the Australia-based Armament Research Services.
The images of the firearm showed that an electrical wire passed through the cap on the end of each pipe.
That pointed to the use of an electric firing mechanism, Jenzen-Jones said.
"The electrical initiation method (...) was likely selected in this case as conventional cartridges are much harder to acquire in Japan than in many other regions," he added.
There have been several cases in recent years of people arrested in Japan for illegally making firearms.
In 2018, police arrested a 23-year-old man in the western city of Himeji for making a gun and more than 130 bullets at home. Also that year, police detained a 19-year-old university student in Nagoya city for manufacturing explosives as well as a gun with help of a 3D printer.