Here is a list of every buff in my mod scripts and an explanation for why I chose to make that change.

In general, I wanted every weapon to be viable, fun, and unique without straying too far from the vanilla experience. Balancing weapons without any nerfs is tough, but I did my best.

Things to note:

• Many numbers are approximate and/or rounded.
  
• DPS stands for damage per second.
  
• DPM stands for damage per magazine.
  
• Accuracy numbers are presented in this order: crouching spread / standing spread / running spread -- smaller numbers are more accurate.
  
• For shotguns specifically, accuracy is written as vertical spread x horizontal spread.

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The assault rifles were already the gold standard for unique, distinctive weapons that don't feel like clones of each other. The problem is that the vanilla AK47 is the best weapon in the game, and it overshadows the other three rifles.

DPS: 447 - DPM: 2,320
No changes.

DPS: 433 (was 376) - DPM: 1,900 (was 1,650)
The M16 is neither overpowered nor underpowered, putting it really close to the "Goldilocks Zone." However, I did give it a damage buff to compete with the AK47.

• 15% more damage (33 > 38)

DPS: 398 (was 365) - DPM: 2,880 (was 2,640)
The SCAR is arguably the worst assault rifle, mainly because its atrocious reload speed drags down otherwise decent stats. I've buffed the reload speed, damage, and accuracy to compensate as best I can without nerfing other rifles.

• 10% more damage (single 44 > 48) (burst 132 > 144)
  
• 15% faster reload (3.35s > 2.85s)
  
• 15% more accuracy (0.05/0.35/4 > 0.04/0.3/3.5)

DPS: 461 (was 400) - DPM: 1,900 (was 1,650)
The SG552 shares most of its stats with the M16, and I kept that in place because I think it's already a "sidegrade" to its sibling. The scope and higher fire rate are balanced out by having the slowest reload of any assault rifle.

• 15% more damage (33 > 38)
  
• 15% faster reload (3.45s > 2.9s)

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In contrast with the assault rifles, the sniper rifles are atrociously balanced. Some are obviously better than others, and two of the rifles don't feel particularly unique to use either. I've overhauled all of them to feel more distinct and viable.

DPS: 436 (was 360) - DPM: 1,980 (was 1,350)
The vanilla Hunting Rifle is so thoroughly outclassed by its military counterpart it's often considered a Tier 1 weapon in spirit. It has received major buffs to make it a true Tier 2 weapon with a knack for quickly picking off Special Infected. The faster fire rate is addicting!

• 33% larger magazine (15 > 20)
  
• 20% more reserve ammo (150 > 180)
  
• 10% more damage (90 > 99)
  
• 20% more accuracy (0.1/1.5/3 > 0.08/1.2/2.4)
  
• 10% faster fire rate (4/s > 4.4/s)

(Side note: I really wanted to buff this gun's reload, but it's the only weapon in the game that has animation glitches if you change the reload speed. Ugh...)

DPS: 360 - DPM: 2,700
No changes.

DPS: 151 (was 117) - DPM: 1,890 (was 1,575)
The bolt-action sniper rifles are generally considered to be worse than the Tier 1 weapons, so I gave the Scout massive buffs and made it an actual Tier 1 weapon. It will now spawn wherever low tier weapons can be found, and it's great for picking off Spitters and Smokers.

• Downgrade from Tier 2 to Tier 1
  
• 20% more damage (105 > 126)
  
• 10% faster reload (2.9s > 2.6s)
  
• 25% more accuracy (0/0.1/3 > 0/0.075/2.25)
  
• 10% faster fire rate (1.11/s > 1.2/s)

DPS: 266 (was 109) - DPM: 5,060 (was 2,300)
It used to be the worst gun in the game, but the AWP has been given a compelling new role as a Special Infected killing machine. Even without headshots, it can kill Boomers, Spitters, Hunters and Smokers in a single hit. It's still bad against hordes of Common Infected, but now a single bullet does as much damage as a shotgun blast.

• 220% more damage (115 > 253)
  
• 10% faster reload (3.65s > 3.3s)
  
• 25% more accuracy (0/0.1/3 > 0/0.075/2.25)
  
• 10% faster fire rate (0.95/s > 1.05/s)

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The shotguns are balanced, but way too similar to each other for my liking. I sought to give them more distinct roles by exaggerating their differences -- now, the Wood and Benelli shotguns are even better at killing Common Infected hordes, and the Chrome and SPAS shotguns are best against Special Infected.

DPS: 1,012 - DPM: 2,783 (was 2,530)
There's not much I can do to make this gun unique without nerfs, so I gave it a larger magazine to emphasize its role in killing hordes.

• 10% larger magazine (10 > 11)

DPS: 1,188 (was 1,008) - DPM: 2,970 (was 2,520)
The SPAS is now a mid-range powerhouse thanks to higher damage and accuracy, but the buffs to its counterpart make it slightly worse against Common Infected by comparison.

• 15% more damage per pellet (single 28 > 33) (burst 252 > 297)
  
• 35% more accuracy (3.5 x 8 > 3 x 5)

DPS: 288 - DPM: 2,250 (was 2,000)
This pump shotgun was adjusted in the same way as its Benelli autoshotgun counterpart, with a larger magazine to help against Common Infected.

• 15% larger magazine (8 > 9)

DPS: 340 (was 285) - DPM: 2,368 (was 1,984)
The newer and shinier pump shotgun is the Tier 1 equivalent of the SPAS: a highly accurate Special Infected killer that does worse against hordes due to a comparatively smaller magazine.

• 20% more damage per pellet (single 31 > 37) (burst 248 > 296)
  
• 10% more accuracy (3 x 5 > 2.5 x 4.5)

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The vanilla SMGs are a bit of a worst case scenario for balance: there's a clear order of best to worst and they're all extremely similar to each other. I decided to make them much more unique by taking cues from the design of the assault rifles.

DPS: 320 - DPM: 1,200 (was 1,000)
The Uzi has been adjusted to be similar to the SCAR: a highly accurate weapon with a huge magazine, balanced out by relatively low damage.

• 20% larger magazine (50 > 60)
  
• 30% more crouching accuracy (0.7 > 0.5)

DPS: 400 - DPM: 1,250
No changes.

DPS: 387 (was 347) - DPM: 1,450 (was 1,300)
Since I can't nerf anything, I gave the MP5 a few simple buffs to keep it competitive with the Silenced MAC-10 and the newly buffed Uzi.

• 10% more damage (26 > 29)
  
• 15% faster reload (3.1s > 2.6s)

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The secondary weapons are all very unique, but again, there's an obvious hierarchy in viability. There's only so much I can do with my scripting skills, but I've tried to make every secondary option a compelling choice.

DPS: 207 single, 414 double (was 166, 331) - DPM: 720, 1,440 (was 540, 1,080)
The Pistols are often considered some of the worst weapons in the game, being thoroughly outclassed by melee weapons up close and the Magnum at a distance. I've given them major buffs so going guns akimbo feels as cool as it should be.

• Removed accuracy penalty when incapacitated
  
• 5% larger magazine (15 > 16)
  
• 25% more damage (36 > 45)
  
• 20% more standing accuracy (1.5 > 1.2)

DPS: 267 - DPM: 640
No changes.

There's unfortunately not much I can do to change the melee weapons with a simple script, but I decided to give blunt melees a slight buff just to even things out a bit.

• Blunt melee friendly fire damage lowered to 5 on Expert

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The cruel irony of Left 4 Dead 2 is that the heavy "Tier 3" weapons are generally worse than standard primaries despite their rarity. I've adjusted all of them to be worth using so players will actually be excited to find them.

DPS: 168 (was 100) - DPM: 400
This hunk of junk has had its reload speed nearly cut in half so it's actually an effective weapon. It's very fun to use now, if you can believe that!

• 40% faster reload (4s > 2.4s)

DPS: 455 - DPM: 9,000 (was 7,500)
The M60 was already a good weapon, but it runs out of bullets too quickly to justify picking it up sometimes, so I've increased its magazine size a bit.

• 20% larger magazine (150 > 180)

No changes.

This heavy turret has an absurdly long cooldown time in vanilla L4D2. Meanwhile, due to what is almost certainly a typo, the Minigun has a cooldown time of 3 seconds when it overheats. Since nerfs are illegal, this cooldown time was gutted. Note that the cooldown is intentionally slower than the Minigun's because this turret does more damage.

• 80% shorter cooldown (60s > 12s)

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I also made some miscellaneous changes to things that are not weapons. I would have done a lot more, but there's only so much you can change in the game's code, unfortunately.

As much as I love Adrenaline, I have to admit that 90% of the time, the speed boost is too niche to make up for the paltry amount of health it provides. Therefore, I increased Adrenaline's health boost to 30 so it's a more compelling choice compared to the Pain Pills.

• 20% more health restored (25 > 30)

Molotovs are better at killing zombies and Bile Jars are better at distracting them, leaving the Pipe Bomb without a niche as the weakest throwable item. I decided to simultaneously buff its killing and distracting potential by increasing the fuse/beep time to 8 seconds.

• 33% longer fuse time (6s > 8s)