Basically, all weapons are bought, except for a couple, such as a flare gun and an RPG, they are obtained from achievements (pass tests), from the starting ones, the best options are the quick silver and the sawed-off shotgun (I advise you to bring the quick silver to 0.0 reload time, you will get a strange fire rate, like minigun), the price of each varies from 600 to 1800), to get an advantage of the elements you need either an elemental gun or elemental bullets for it.

everything is simple here, even too simple, you get them from red chests and in the store, from yellow ones only skills drop on the character, most of them are available for any weapon, and now for 3 individuals: accuracy, spread (+???) and explosion radius, even if you kill me, I don’t understand the point of PLUS spread.
Accuracy - for all weapons except shotguns, sniper rifles and explosives.
Spread - for shotguns and that’s it.
Blast radius for rocket launcher, grenade launcher and auto launcher (flare gun is considered a pistol).

Milestone mods are improvements offered in 2 choices after increasing the level of the weapon (after 2, 3, 4 regular improvements)
Most often, again, they are bought, of course, there are initially available ones.
I will say in advance, the ricochet does not extend the shooting distance, although it is legendary, but there is little use, but shrapnel is a powerful thing, it deals 50-100% damage to all enemies nearby when killed with one shot, it is especially good at shooting at ticks.
Since version 1.02, shrapnel has become a legendary mod.
And now about the pain in my understanding, I spent 10+ hours trying to find the missing modifier and already asked for help from the developers, they told me that it was for SMG, and apparently I was just unlucky that day, spray and pray.

why the hell did you have to make each armor separate? Because each level ADDS your armor value, and does not improve the % absorbed by it, if you already have armor, it will be replenished
The missing ones are obtained from the achievement with the ice upgrade.
Important bug, please note: If you buy the last elemental upgrade in the store - you will NOT be given an achievement either in the game or on Steam, the developers know about this.
The easiest way to get the fire upgrade is in the flare gun test.
Small tips for getting elemental upgrades: play on a doctor or shooter of the maximum level, Linda will slightly reduce the chances, and the commando will almost kill them because of the number of upgrades. In chests, try to get rid of single-level upgrades so that they do not come across again, loot in chests cannot be rerolled. At the start, it is very important that the store is immediately after the mission, sell the first 2 or 3 chests so that you have enough for a weapon if you get one, if you don't get one, try to upgrade the weapon and get elemental bullets. Take each container and try to improve them to gold, it is advisable to complete with 4+ duration, do not forget about improving 5 choices for 1 task and of course it remains to pray.

And now skins other than the basic one:
For lvl 10 and for the support pack.
Starting skills:
1. Each mission gives you a pack of ammo in the 5th slot, like the others.
2. Increases the base ammo of all weapons by 30%.
3. 25% discount on machine guns and shotguns in the store.
4. When falling (death), you throw a grenade under yourself (F teeth), by the way, you can find this perk in the store or chests.
5. Starting the game in armor. (nice though)
And now perks:
(The fire grenade is very weak and even against a crowd it works questionably)

Skins other than the basic one.
For lvl 10 and for a support pack:
Starting skills
1. Increases stamina by 10.
2. Certain consumables are shown on the map.
3. Each task gives you an energy drink in the 5th slot, like the others.
4. Get a chance to reflect a melee attack.
5. When killing with a shovel, you get +10% damage for 10 sec, stacks up to 5
Perks:
(to enlarge, click on the picture)

(I hate meta)
Weapon: quicksilver - it's enough to simply bring the reload to 0 and increase the damage and you will essentially have a minigun, the rate of fire = click speed.
The sawed-off shotgun is also a good choice, especially if you find electric bullets for it. It is advisable to keep the reload below a second and 3 rounds in the magazine, the rest is in damage and the amount of ammunition, sharp and electric bullets are very good (the rest of the elemental ones are also good) and a expanded load.
Freedom - cheap and cheerful, or rather fast and cheerful, I remind you that all weapons have weight and the pistol has the lowest, very good for speedrunning with Linda, good ammo, average damage, it is better to take frost bullets for it.
The auto launcher itself is a good gun, you can roll damage and reload speed.
Thundershot, a good pistol with high damage, you can add ammunition and reload speed + magazine volume.

Now skills:
It is better to always take HP, a powerful thing itself, if you play solo and speedrun, then speed and additional stamina + regeneration will go very well, as well as a healthy heart, in such a build the adrenaline rush will help, it will go very well on Linda because of her RMB, if you also pick +1 to HP on hit, then you can live for a long time.
In general, armor and the desperate strenght also show themselves well.

For characters:
Commando - Take additional damage when picking up ammunition and additional ammunition with 2 identical weapons, I advise you to take pistols or shotguns, well, or quicksilver with a sniper rifle.
The radius of the grenade and rapid fire will also help. I remind you that the grenade can be thrown further by holding down the RMB.
Linda - I described above. If you shorten it, then take a bunch of stamina and +1 HP on hit and just walk around and beat everyone with RMB, you MUST shoot at spitters and speedrunners, sometimes they bug, and spitters are just unbalanced.

Thank you for reading the guide, on the first version it took about 45 minutes, on the second add-on half an hour, I will add and change the screenshots as I open new perks and characters, then I will also throw in a view of the characters with skins
if possible, I ask you to like the guide, if you want you can send it to friends, save it to yourself and give rewards!