It took about five or six viewings of TENET before I understood it all.
Here's how I visualise reverse ammo in my head
TIME MOVING FORWARDSYou shoot a bullet into a wall - - - - - > The bullet will "always be there" - until removed or the hole is fixed...
TIME MOVING BACKWARDSThe bullet hole in the wall has 'always just been there' - - - - - - - - - > until someone shoots the bullet into the wall going forwards.
To someone who is inverted - the bullet hole will just always be there and always will until un-fired into a gun. I don't think that the bullet hole just appears as soon as the ammo is
inverted but i've not thought about it that much.
Also - at the opera - when Neil runs past The Protagonist and un-shoots a bullet into a gun. The only way I can rationalise this in my head is - if he ran past Protagonist,
fired the gun intending to shoot the bullet through the SWAT guy into the wall - but since the ammo and probably gun too - were both inverted - IT APPEARED to run backwards
to US. ( Neil was not inverted in that scene as after that - he runs down the stairs forwards, not backwards ).

Finally - the interrogation scene at Freeport in Tallinn - it confused me for AGES - how - you see Sator invert and leave the Freeport with Kat - but then TENET show up and Kat is still in there.
Then I realised that when Sator is about to shoot the Prot - TENET burst in and Sator escapes a few seconds into the past. So from TENETs POV he vanishes as theyre still moving forwards.
Sator emereges a few seconds into the past on the blue side and interrogates ( a now backwards Protagonist ) - BUT - because Kat herself was NOT inverted.. She is STILL IN THAT ROOM
in the present - for TENET to rescue.
I might watch it again as I didn't fully realise that literally after Protagonist inverts and causes the car crash he witnesses on the Freeway - the film then goes backwards all the way
to Sators yacht in Vietnam - which happens the exact same time as the battle at Stalsk 12 ( which Michael Caine tells Prot about in the beginning ) AND the Opera siege.
So the film starts and ends at the same place.