So after a little more discussion I've figured out how to add sound playback to the breakable, so the full setup looks like this:

Func_wall
takedamage = 2
health (whatever you want it to be)
armortype/armorvalue (whatever you want it to be
noise (whatever sound you want the brush to make when you hit it but don't break it)
th_pain = train_wait (plays sound in noise field whenever you hit it)
th_die = SUB_UseTargets (targets 2 separate info_notnull's that play the sound you want to play when the object is broken and removes it, respectively)
target = the name of the 2 notnulls

Notnull 1
targetname = whatever you put in the target field of the breakable
noise = the sound you want to play when the breakable breaks
use = train_wait (play the sound when triggered by the breakable dying)

Notnull 2
targetname = same as the first one
killtarget = name of the breakable
use = SUB_UseTargets (remove the breakable when this gets triggered by it dying)

Please feel free to correct me if something I put is incorrect, I'm still pretty new to q1 mapping and map hacks especially. But map hacks are really fun and I've almost become more interested in them than actual mapping at this point (but not completely though) :P I'm also trying to think of how to add rotation to a turret hack I made last week but I haven't really checked any of the quake c sources related to rotating functions to see which ones I might be able to use 

That's a really well constructed and described hack. Welcome to the thread!

In terms of rotation, a warning and two pointers. First warning is that standard Quake just doesn't do collision on rotation. The amount of highly specialised code that mods apply to get even crude approximation of rotating solids, I wouldn't expect a hack to be able to get anything like collision working.

If you just want visible rotation things are better. There are already suggestions in this thread about how to use avelocity for simple looping animations. If you're looking for something more triggered, you might want to look at SUB_CalcAngleMove. Although any entity which used the function got cut from the final release of Quake, it's still available for hacks to use.

Alternatively, you might want to look into the AI functions if you'd like rotations that follow the player. You need to set quite a few keys on an entity to trick the game into running some of the built-in AI stuff for turning to face an enemy, but it should be possible. More advanced stuff, but something to get your teeth into... 

Having your breakable also create some info_notnulls using external .bsp models for rubble and have a movetype 6 would be cool too. Can't remember a good way to give them starting velocity at the moment though and still be triggered on break. 

Thanks :) I got the idea for the armor from one of your articles https://tomeofpreach.wordpress.com/2013/05/11/trigger_damagethreshold-simulator/#more-974 I may add it to the turrets too if possible. Speaking of, I will definitely look into the AI turning/rotating functions to see what I can do with them 

How can I make worldspawn shootable and killable?
Perfect accuracy gameplay challenge... 

So, I was idly tugging my way through some of the vanilla QC files, and noticed functionality I was hitherto unaware of!

In the blurb for the trigger_once entity, it says this:

if "angle" is set, the trigger will only fire when someone is facing the direction of the angle. Use "360" for an angle of 0.

The code to do this seems to be in there, but I cannot recall a single time in the id1 levels where I noticed this behaviour.

Is this used anywhere in id1, and if so where?

Yours curiously... 

one place is on start.bsp
those 'this hall selects foo skill' centerprinting triggers only display their messages when you're facing towards the teleports. walk backwards into the teleporters and you won't see the messages at all. 

768_negke.bsp
sm179_otp.bsp 

I didn't bother to dig into the whole backlog of messages here, but I made a (fairly simple) hack that allows for killable "static" entities. I've tested this with flames, but not with other things, I suspect that light globes may work.

step one, make a trigger under the player spawn and make it target an info_notnull called "flame". Make sure this info_notnull is some distance from the player, as touching it too early will crash your engine.

Place a "light_flame_large_yellow" somewhere in the map to precache the model.

give the info_notnull the following keys:

targetname flame
model progs/flame2.mdl
frame 1
use func_wall
think PlaceItem
nextthink 2
mdl progs/flame2.mdl


This should now allow you to killtarget the flame entity. Note that static ents don't occupy the edict list, however this hack will. I guess it will also allow for a touch function to be run on it after this point too.

One final thing to consider when using this hack, touching this entity before its had its "nextthink" even by a monster will crash the game. Even shooting the flame will crash it. I suggest spawning them well away from players and monsters.

The nextthink function forces the notnull entity back to a non-solid non-bsp state whilst retaining the flame model. In the example above this happens 2 seconds after it spawns. 

(oh, the trigger should be a trigger once) 

those 'this hall selects foo skill' centerprinting triggers

Blimey, I didn't have to go far then to find one. 

That's a fair amount of work for a killable flame. Rather than messing with thinks and nextthinks and all that, you can just set the modelindex of an info_notnull to that of the spinning flame model (flame2.mdl, check in the console first to see which slot it takes of course) and have a simple displayed model that you can killtarget like normal, no extra futchery with crashing when touched or anything. The flame will automatically do its snazzy little spinning and burning animation, similar to a backpack, and the frame field changes the size with 0 being the smaller flame model (light_flame_small_yellow) and 1 being the larger version. Old example webm here. 

the flame models are static and don't appear in the edict list though? 

They're still given a modelindex, no different from the player or enemy models or worldspawn/brush models. Not sure how to find which model has what number in Fitzquake-derived engines but Darkplaces (ugh) has a handy modellist command that will print out every model currently loaded by the game in order. Worldspawn comes first, then count the list by one. Worst case scenario, experiment a bit until you find the right modelindex and remember to place things in the right order that enemies/health changing by difficulty won't break the hack. 

You might have to make separate entities for different skill levels, with each having the correct model indices for the corresponding skill levels. :/ 

Moving the static torch you're using to precache the model as near to the top of the entity list as possible will help to stabilise the modelindex across all the skill levels in your map. Also worth remembering that each time you add a brush entity, that adds one to all the precache modelindex values. So you want the map to be basically finished in that respect before you finish off the hack. 

Hey guys, back for another update. My rotating turret hack is almost 100% done :D I just need to take a few minutes tomorrow to iron out some little kinks related to the offsetting of the body model in relation to the shots and it will be good to go and I can make a demo map and full write up on it. In the meantime here is a weirdly rotated screenshot from QuakeDroid showing it in action in a crappy little box room :)

https://imgur.com/a/IeLhHFe 

Eagerly looking forward to seeing it in action. 

Nice that a "newer guy" can make cool stuff! 

If what he's posted in #tf and related channels in such short time is any indication, rekta is a new maphax savant. 

I mean it's not that impressive, you can do it easily if you have basic programming knowledge and take the time to read the sources to know what fields and functions you can and cant use where... Or in my case, try and figure out why entire segments of code were replaced and moved elsewhere but still in the original files commented out, then get confused when what you want to do isn't working the way it should :D it's not like I'm making Quake Rally 2 or something like that... Though I do like racing games so maybe I'll give that a shot some day ;) 

Ok, so, here is the write up on the rotating turret hack. Now my setup probably isn't the best but it works fine, at least in SP, so I'll probably keep using it in maps going forward anyway. It's not too difficult to set up but there are several steps involved. Also apparently this is too long to put in one post since the character limit is 5k and I have more than that :O So i will need to divide it into multiple parts.

So, here is how you do it. First at the map origin you need to set up the model of the turret itself. Of course I suppose you could use a custom mdl but these are vanilla maphax we are talking about here :) Now after you make it you might need to move it up a little depending on the type of projectile you want to use so it doesnt look like complete crap when it shoots, I used hknight_shot so I had to move the model up a few units to get it to look right. After you made the model you need to turn it into a brush entity so it will have a model and modelindex value, I used func_wall. And of course you should encase it in a box to prevent leaks and add appropriate lighting. Then add a info_notnull in the spot where you want the turret to be and give it a target name like "turretmain" or something, it will be the main turret body. Now you will want to copy the main model to the notnull, you do that by setting the modelindex and model fields to the same model and modelindex values of the turret model you want to use, if you want to have multiple different styles in your map you need to create multiple models. You can find these values using the "edicts" command in the console, if you want to be able to find the edict numbers of the turret models easier just cut and paste them to the top of your .map file just under worldspawn, that should make things a little easier since now they should be closer to the top of the edict list. In my case the model was *1 and modelindex was 2.0, so I used those. So so far you should have something that looks like this

info_notnull
.....origin = whatever the position is that you placed the notnull at
.....model = *1 (or whatever the model number of your model is)
.....modelindex = 2.0 (same as above)
targetname = whatever you want the targetname to be i guess.

Now if all things went well when you go into your map you should see your turret in the spot where you placed it. Good. But, it doesn't shoot obviously, or rotate either :( SO we need to fix that. Unfortunately I couldn't figure out a way to get any "smart" player detecting functions working so it can only really reliably track 1 player meaning this hack is technically SP only for now. But that's ok, you can just make multiple copies that track specific players, no big deal. First add an "enemy" field and set it to 1, this will cause it to track the first player since they are always the first entity loaded in a map. Now let's get the rotation part out of the way first. For the rotation to work properly we need two things, we need to add a "yaw_speed" field to the turret that contains the speed that we want it to rotate at, and a function that consistently sets some specifc values, specifically ideal_yaw and angles_y. Luckily there is a function that does just that for us, ai_face. So we need to put that somewhere, but where is a little tricky. We can't put it in think because neither ai_face or ChangeYaw (the helper function that ai_face calls that actually rotates the model) resets nextthink meaning it would only run once which isn't exactly ideal. There are 2 options that we as the mapper can consistently control though, use and th_pain. So we can put our rotation function in one and the shooting function in the other, it doesn't really matter which one since the pain state of the turret will need to be activated by a use'd entity shooting at it anyway. So I put ai_face in use and the shooting function in th_pain, in my case hknight_shot. So now it should look like this: 

.....origin = whatever the position is that you placed the notnull at
.....model = *1 (or whatever the model number of your model is)
.....modelindex = 2.0 (same as above)
targetname = whatever you want the targetname to be I guess
yaw_speed = the speed that you want the turret to rotate at
enemy = 1 (or whatever the player number is that you want it to target)
use = ai_face
th_pain = whatever shoot function you want the turret to have, only *MONSTER* shoot functions can reliably be used as far as I can tell since I haven't been able to find a function that will reliably set the values required for you to be able to use player weapons effectively, you can still technically use them, you just can't constantly re-aim them so they will either always shoot in one direction or shoot wherever the player is pointing at the time, depending on how you have it set up. Who knows, maybe that's what you want and you can find a use for that, but this "tutorial" of sorts is about hostile turrets that attack the player, not shoot in the direction they are facing ;) Oh yeah, and the monster whose shoot function you want to use needs to be precached in the map first.

Now as you can probably imagine, we need to set up some triggers so that the turret will do it's thing. So make 2 identically sized trigger volumes that take up the exact same area that will correspond to the line of sight of the turret, of course they can be multiple brushes if you want a concave shape, it just matters that they are targeting the things they should be. One of the triggers should target the turret itself, which when the player steps on it will cause the turret to rotate since its use function causes it to do that. The other trigger will target a new notnull that we need to add in the exact same position as the turret one which is supposed to trigger the turret's pain state, causing it to shoot. It will do that by being inside the body of the turret and constantly shooting it with it's own projectiles. Of course for the turret to be in pain it needs to have health, so you should add a health field to it and set it to whatever you want. You also need to set "armorvalue" and "armortype" as well, so that the turret won't kill itself by being shot by its pain trigger. If you want it to be destroyable you should set the armortype to soak up exactly as much damage as the triggering projectile will give (a tutorial on how to do that is given by Preach here https://tomeofpreach.wordpress.com/2013/05/11/trigger_damagethreshold-simulator/) and make sure that the player will have a weapon at that point that does more damage than that. Finally, you need to add 3 more fields, "solid", "takedamage", and "movetype", and set them to 4, 2, and 7 respectively. The solid and takedamage are set so that it will, you know, be able to be shot and take damage, and movetype 7 (PUSH) is required by solid 4 or else the map will crash.

So now that the turret is all set up to get shot and in doing so shoot at the player, now we need to add the notnull that will be firing those trigger shots. This is suuuuuper easy. just add an info_notnull in the exact same position as the turret and give it a targetname, in my case "turrettrigger", a use of hknight_shot, and an enemy of whatever the edict number of the turret's notnull is, in my case 7. Now when it is triggered, it will shoot at the turret, which will cause it to go into "pain", which will cause it to shoot at the player since that's what it's pain function is set to do! And it will rotate as well assuming you have another trigger covering the same area targeting the main turret itself.

And thats about it! Now you should have a turret that, when triggered, shoots at the player and rotates to track them! The final setup looks something like this:

notnull 1 (turret)
.....origin = whatever the position is that you placed the notnull at
.....modelindex = modelindex of turret model you want to use
.....model = same as above
.....targetname = whatever you want the targetname to be I guess
.....yaw_speed = the speed that you want the turret to rotate at
.....enemy = 1 (or whatever the player number is that you want it to target)
.....use = ai_face
.....th_pain = whatever shoot function you want the turret to have, taking into account the restrictions outlined before
.....health = whatever
.....armortype = minimum value required to negate damage from triggering projectile but not from more powerful player weapons
.....armorvalue = a number high enough to where it wont go down when it gets shot, i believe 999999999 is a good amount, Preach has an article on this topic)
.....solid = 4
.....takedamage = 2
.....movetype = 7

notnull 2 (turret shoot trigger)
.....origin = same as the turret itself
.....targetname = whatever
.....use = hknight_shot
.....enemy = entity number of turret 

Here is a link to a zip containing a small crap box demo map showing the turret in action and the map source https://drive.google.com/file/d/1_2U4tXHpkpqdhfZi-3J1PyZVtOoFXp_r/view?usp=drivesdk I would have taken the time to make a somewhat better demo map but this took a little longer than I expected to write since I suck at trying to explain things lol, so I just decided to throw out the map I used for testing instead of making a somewhat more realistic use case environment. As usual if you have corrections or tips for improvements feel free to point them out :) 

Haven't read this yet but GG. Grabbing my reading glasses.