I finally got it to work!!!
It appears, that "volume" (inside SOF2.wpn file) is NOT the only parameter that affects detection by enemy NPCs when firing a weapon - another factor appears to be the "DamageAmmoType" parameter inside SOF2.poses file:
Changing the value for "9mm" (which is used by both Micro-Uzi and MP5) from "RapidBullet" to "Bullet" (just like is the case for all the other projectile weapons) finally enables Stealth for the MP5 - the other NPCs won't notice when a fellow NPC gets hit by MP5 bullets and also no alarm will go off when hitting a NPC with MP5 bullets! At least, unless the NPC actually sees you - but this was also already the case for the suppressed SOCOM pistol!
*HOWEVER* that achievement gets by far offset by the fact, that changing this value almost completely f***s up death animations and death screams - not only for the MP5 or Micro-Uzi, but for all weapons in SoF2! It's almost the same effect, as if one had disabled "Death Animations" in the Violence Lock menu - blood, splatter and dismembered limbs still work properly, though.
But first things first: how did I come to the finding described above? Well, I did a couple of experiments and observations:
- firing the unsuppressed SOCOM pistol causes the NPC to notice you and the alarm to go off, as one should expect...
- firing the unsuppressed SOCOM pistol causes a death sound/"scream" when doing a headshot to a NPC - regardless of the running/moving speed!
- firing the suppressed SOCOM pistol does not cause a death "scream" when doing a headshot on a NPC (the same applies when doing a slice with the knife on a NPC head) - the NPC just silently falls to the ground.** Except...
- ...when firing the supressed SOCOM pistol while running or moving fast towards an enemy, then (only then!) the supressed SOCOM pistol also causes a death "scream" when the NPC is hit on the head.**
- I just recently figured out, that when shooting the MP5 (after having reduced the "volume" parameter to 0.1 or 0.0), actually no NPCs become alerted and no alaram goes off - unless you hit a NPC with an MP5 bullet. Even if the hit instantly kills the NPC, the other NPCs will still become alerted and the alarm will go off. But unless you actually HIT a NPC with the MP5, you can literally (I tried it out!) shoot it just a few inches away from a NPC's head and despite the bullets literally swooshing inches away from his ears, he just won't bother and also no alarm is raised!
- With this new insight, it dawned upon me, that the NPC noticing the player when shooting a weapon must have an additional mechanism that has to be present (or not present) !
- I also did another experiment: I lowered the "volume" parameter for the SOCOM pistol to 0.1 (default is 0.75) and later also for the M1911A1 pistol to 0.1 (default is 0.75) and even for the M4 rifle to 0.1 (default is 0.8).
The outcome of this experiment confirmed my assumption: now, even the unsuppressed SOCOM and the M1911A1 pistol and even the M4 rifle do not cause the NPC to notice you or raise alarm! - despite creating a loud shot sound unlike the MP5(SD) or suppressed SOCOM pistol! Also, hitting a NPC with a headshot with these weapons still doesn't cause the other NPCs to be alerted or the alarm to be raised despite the loud shot sound - unlike the MP5! (and most likely, also the Micro-Uzi)
- Reducing the "volume" parameter also caused the NPC not to sound a death "scream" when getting headshot by these loud weapons - unlike before, when the default value for this parameter was 0.75 (pistols) and 0.8 (assault rifles) !
- But when the "volume" trick works even with the M4 rifle or the M1911A1 pistol, whom both are not suppressed - why does it not work with the MP5!? Thats's, when I came to the idea to dig on an entirely different weapon property!
- After a bit of digging, I noticed that *all* ammo types (0.45 ACP, 5.56mm, etc.) in the SOF2.poses file have the value for the parameter "DamageAmmoType" set to "Bullet" - the only exception is "9mm", which is set to "RapidBullet". Since 9mm is only used by the Micro-Uzi and the MP5, this must have been the trouble-maker since all the time - and indeed it was: changing the parameter from "RapidBullet" to "Bullet" immediately solved the MP5 Stealth issue... now, the NPC does not notice the player even when getting hit by the MP5. Of course, unless the NPC (even just one of them!) actually sees you for even just a split-second!
- Everyone, who has completely played SoF2 singleplayer and tried out all weapons (including the MP5 that came later with patch 1.01) knows, that both the Micro-Uzi and the MP5 cause the same type of unique death animation on a NPC: that Hollywood-style (or Gangsta-style) NPC twitching without falling to the ground as long as the NPC keeps getting hit with MP5 or M-Uzi bullets before finally falling to the ground.
- Since both the MP5 and M-Uzi use the same ammo type ("9mm"), this unique death animation seems to be tied to 9mm only.
- Unfortunately, this achievement is of no use (at least for me), because it almost kills the "soul" of every SoF game: the death animations, which are an essential (and often hilarious) part of the violence of the game - maybe even MORE important than the blood, splatter and dismemberment effects. And worse than that, not only for the MP5/M-Uzi but for ALL the other weapons, too! I want stealth with stealth weapons (MP5, suppressed SOCOM pistol, knife) in SoF2, but I definitely do not want to sacrifice the very core of every SoF game for that...
**This is not a new or special "finding" - I noticed that from the very beginning since playing SoF2, over 15 years ago...
Keywords, Aspects and other Findings, that might help in completely solving this problem: (I do not consider it solved yet!)
"volume", "visibility", "impact", "flesh", "socomfire01", "socomfire02", "unsfire01", "unsfire02", "AmmoType", "DamageAmmoType", "Bullet"/"RapidBullet", Death Animations, GHOUL2 animation system, "carcass.exe" (the program used for/by GHOUL2 animations), internal cvars and dependencies regulating, or more precisely: enabling/disabling the specifics ("Blood", "Death Animations", "Dismembered Limbs") of Violence Lock - I do not mean "lock_blood" with that!
- From what I know so far, the data/settings for weapons animations is stored in SOF2.inview file and the weapons animations files themselves are stored as .glm, .gla and .frames files in "models.pk3\models\weapons\[weapon name]\" and "therest.pk3\skeletons\weapons\[weapon name]\" , respectively - and where is the information stored for the death animations??
Still looking forward for any advise or support...