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A readied action triggers the moment the trigger happens. As the sniper i am going to shoot either of those guys in the back with weak weapons and light armor if it looks like they are getting magic shifty. The spell caster starts getting "magic shifty" triggering the sniper into shooting him in the kisser. The spellcaster is not guna be a bruiser charging into combat with a huge/glowing sword nor is he going to be the guy hefting a huge gattling gun or the guy that you did not notice and that is most likely to get some back staby action in on the sniper.because the sniper is focused on the people he thinks is a shifty magic user.
Or you can take what the crb has to say on it.
Normally, you can concentrate even in a distracting situation, but if you’re casting a spell and ====>you take damage from either a successful attack that targeted your AC <==== or from an effect that you failed a saving throw against, the spell fails.
You are most at risk of taking damage while casting when a spell’s casting time is 1 round or longer, you have provoked an attack of opportunity, ====> or a foe readied an action to attack you when you began to cast.<==== However, if you are taking ongoing damage (such as if you are bleeding or on fire), your spells are not disrupted in this way.
Sadly, the relevant bit of the rules in this situation is in the Tactical Rules section of the CRB.
If your readied action is purely defensive, such as choosing
the total defense action if a foe you are facing shoots at you,
it occurs just before the event that triggered it. If the readied
action is not a purely defensive action, such as shooting a foe if
he shoots at you, it takes place immediately after the triggering
Shooting at a spellcaster is clearly not a purely defensive readied action, therefore the readied action triggers after the spellcasting action has been completed.