Reactions and triggering actions order of resolution

So specific scenario-a group of crossbowmen guards have a readied action to shoot at a spellcaster as soon as they start casting a spell

The spellcaster starts their spell which has a casting time of 1 action

is the spell completed before the attacks occur or do the attacks happen before the spell is completed? i.e. if the attacks drop the caster to 0 hp does the spell go off? If they hit, is there a chance they can disrupt the spell (I.e. require concentration checks to complete?)  Can the caster take a reaction and cast shield while casting the other spell (irrelevant if the spell was completed before the readied action takes place). 

Per the rules (I think) any reaction, unless otherwise stated, happens after the trigger. In this case, the trigger is the spell being cast, so the shooting happens when the spell is finished.

Quote from Maruntoryx >>

Per the rules (I think) any reaction, unless otherwise stated, happens after the trigger. In this case, the trigger is the spell being cast, so the shooting happens when the spell is finished.

Well, in the given example, the trigger was "they start casting a spell", which is why it's unclear. 

We know it's possible for a Reaction trigger to be the start of a spell being cast and for a Reaction triggered that way to happen before the spell is finished being cast — that's how Counterspell works. What's not clear, I think, is whether you're allowed to assign a trigger that granular for the Ready action.

For the proposed scenario, I'd rule the same as other folks said in Readied Action, Vortex Warp: Does this work?:

Quote from AntonSirius >>

This wouldn't work, because the attack or casting would be complete before your Readied action would trigger

When the trigger occurs, you can either take your Reaction right after the trigger finishes or ignore the trigger. 

Quote from The_Ace_of_Rogues >>

As a rule, anything that doesn't explicitly say it interacts with a part of the Attack Action- such as Shield, Silvery Barbs, Bardic Inspiration, etc.- can't modify the sequence mid-action. 

Also, if it's useful: Can you ready an action to be triggered after an attack is made but before the damage is dealt?

If you Ready to attack creature as soon as they start casting a spell, i'd rule when the trigger occurs, you can take your Reaction immediately after the spell is cast, not before so.

I disagree with the growing consensus here.  You absolutely can set up a trigger to be able to interrupt the spellcasting process.  As mentioned earlier, that's exactly what happens with Counterspell.  

My main issue with the Ready action in general is that I feel pretty strongly that they are not (and should not be) allowed outside of combat in order to "start things off" with a free attack.  That's not necessarily what's going on in the OP here, but it could be, and if so then that's the aspect of the mechanic that needs to be addressed rather than whether or not it's possible to define a trigger which behaves exactly like the one in Counterspell (since obviously that is indeed possible) . 

To me the reason Counterspell can interrupt the spell being cast is not because of the trigger, but because of the effect on it that say otherwise.

Reaction: In terms of timing, a Reaction takes place immediately after its trigger unless the Reaction’s description says otherwise.

This is a 5e thread from 2022 with a similar question: Interrupting spellcasting

It seems people there reached the same conclusion.

Quote from Plaguescarred >>

To me the reason Counterspell can interrupt the spell being cast is not because of the trigger, but because of the effect on it that say otherwise.

Reaction: In terms of timing, a Reaction takes place immediately after its trigger unless the Reaction’s description says otherwise.

I disagree.  In my opinion Counterspell works that way because of how the trigger is worded:

. . . you take [your Reaction] when you see a creature within 60 feet of yourself casting a spell . . .

As soon as you finish seeing such a thing you can take the Reaction.  It's not as soon as the spell is finished being cast.  It's when you finish seeing a creature casting a spell.  In the OP for this thread, it's been worded in a similar manner.  The trigger is:  "as soon as they start casting a spell".  So, right after a spellcaster finishes starting to cast a spell, they can take the reaction.

By rule, a reaction is an instant response to a trigger that is capable of interrupting another creature's turn immediately after its trigger, whatever that trigger might be.