This represents a complete failure by Abe's "SP" protection team. Even after shots fired, nobody has a gun in their hand (note the holsters visible in some photographs). Everyone is in day-dream mode not paying attention. The guy just walks up and fires two shots with nearly a two second spacing. Two. Whole. Seconds.
Even after this happen - they decide to tackle him! They don't know he's not wearing a suicide belt or has more shots to fire. The whole thing is just stupid. Head will roll.
Outside of US and some select places (India, UK etc, which have active terrorism threats), people don't know gun violence on a day to day basis. Violent crimes involving guns in Japan are very very rare. The response takes a few moments to register.
If a meteoroid suddenly crashed in front of your car while you were driving, would you switch off the engine & inspect the paintjob & fenders? Probably not. It's a situation which you know can happen statistically but you're not prepared to respond in a flash.
Security team gets trained on which threat model? The one that actively persists for the politicians.
In that sense, they would be more on the lookout for a knife attack or some physical assault instead of guns. Things needing proximity. The possession of firearms in Japan is very strictly controlled.
Edit: my dad in his early career worked in counterterrorism ops. He would be prepared to deal with someone bearing a firearm, but would I blame him for a lapse if a person came prepared with nerve agent? probably not.
On the Japanese TV news, the report said about 15 yards - but definitely not in proximity.
With a bunch of cameramen & reporters nearby in a open-air election campaign settings, in a country where private gun ownership is near non-existent, would you immediately suspect something to be a improvised gun?
The cartridges being homebrew with slug shots, the first shot was described more as electric substation going off, some malfunctioning sound equipment etc - not the shotgun type gun report. The second shot around 1.5-2 seconds later had sufficient smoke discharge to be identified as firearm (Information quoted in Wikipedia). The sounds caught on video clip is significantly different for loud sources (e.g. try comparing aircraft engine sound on YT with your recollection of live sound). Hence its harder to know how it sounded like & assess for yourself
Even after this happen - they decide to tackle him! They don't know he's not wearing a suicide belt or has more shots to fire. The whole thing is just stupid. Head will roll.