Also the bullet's effect on kayneth only started when they hit him not the volume hydra something
Incorrect, Kayneth entrapped the bullet in Volumen and it didn't reach him, it only hit the mystic code itself. The bullet that hit his body was the regular contender shot earlier in the fight that punched through the weaker Volumen
One bullet fired when kayneth was using his circuits to the absolute limits. Kayneth has way better circuits then Shirou and Shirou used to damage his nerves on daily basis that's how he did his "training" and Shirou's projection is more akin to summoning once the weapon is treaced it's stored within ubw.
I wouldn't necessarily say using them to the absolute limit, but he was putting more mana into it yes, but that part doesn't doesn't matter here. Regardless of how good your circuits are, you're going to he messed up by the round. Sure you can try to say it wouldn't be instant death, but it might as well be considering your number of circuits isn't going to stop the severing and binding from just destroying them.
The way that part works is it servers the circuits and rebinds them incorrectly, since they're "fixed" it's basically impossible to repair, but more importantly, when the energy still tries to flow through them despite this, it destroys the circuits. Your number of circuits doesn't make this part more or less effective, only the necrosis and nerve damage part.
Shirou being able to damage his nerves on a daily basis is cool and all, but we're talking about an amount of damage that took someone who was likely Touko, or at the least someone near her level, to be able to even attempt to fix, and even then only the hands were partially fixed. You could say Shirou wouldn't take this much damage sure, but simply saying he messed up his own nerves isn't really good enough. Ignoring the physical and nerve damage part, the issue then becomes that Shirou's ability to use magic just dies if he's affected by the round. In which case he has quite literally no way to survive anything Kerry does anymore.
Shirou's projection is still projection, it's just done in a slightly different way so that the World doesn't crush the projections for being things that don't exist otherwise. Once he first projects something, it's stored inside of UBW, this then allows him to project it at any time afterwards as they're registered using UBW, which bypasses the degradation. Every step of the process requires still requires his mana, even the projection of the item into reality still takes mana, it's just less than making an entire functional weapon from scratch would be (which is to say it's an amount that doesn't just kill someone). With this in mind he would indeed be affected if the round touches a projection
