[in progress]
Mohrta has a multitude of weapons with very different firing modes and alt-fires. Most weapons share their ammo pool (color) with two or three other weapons, and once you've ran out of a certain ammo type, you cannot use the weapons from the same color.

I find that it's better to prioritise one weapon of each color and focus on upgrading it. The game gives you plenty of choice, but I prefer to melt down the weapons I don't regularly use due to personal preference and instead use the materials to upgrade my favorite ones.

Each boss gives you a new weapon, which gets classified in the number key of its corresponding chapter, together with the other two of the corresponding chapter bosses (so the weapons of the First World get assigned to the number key 2 of your keyboard, and you cycle through them with that single key).

You have to cycle manually each time you want to quickly change weapons mid-combat, and you cannot set the "default" weapon when swapping to that chapter cycle (it will always be the last adquisition).

Reducing the number of weapons for each "chapter" to 1 or 2 makes it easier to swap mid-combat, and keeps the cycling to a minimum.

Finally, for the less-completionist players like myself, who aren't exploring every corner of the map, melting allows you to improve your main roster of weapons whilst keeping up with the enemy types.

As a personal preference, I don't favor melee weapons in fps games without a dedicated melee combat system. You can use the various melee weaponry if you prefer, but I find that being up close with enemies punishes players and prevents you from dodging most telegraphed attacks.

The Bastard Melee
14 Crystals to upgrade
Your basic melee weapon, decent speed and damage. I keep it mostly for breaking pots and as a backup in case I run out of ammo. It's expensive to upgrade so it's not a priority until late game.

[You cannot melt this weapon and will always be in your inventory.[/i]

Revolver Blue ammo
10 Crystals to upgrade
A hitscan weapon with a 6-bullet chamber. Reloads decently fast. It's fanning alt-fire deals huge amounts of damage and doesn't spend any ammo. A decent pick as a main weapon to upgrade early and keep all game.

[You cannot melt this weapon and will always be in your inventory.[/i]

Trench Gun Yellow Ammo
10 Crystals to upgrade
A 4-slug chamber shotgun with a powerful single shot alt-fire, which you can use to backpedal if you are in the air. Decent damage but suffers from a minuscule chamber, and you cannot cancel the manual-loading animation once it's started. You can melt it since it can be replaced by other Yellow ammo weapons like the Syringe.

Bone Riddler Green Ammo
10 Crystals to upgrade
A 15-bullet magazine uzi. Fires fast projectiles but with unreliable aim, and it's alt-fire is nothing spectacular. It will serve you in the early game till you get other Green weapons like the Deepgun, then you can melt it and never look back.

Larva Launcher Red Ammo
12 Crystals to upgrade
A small 2 grenade magazine makes up for it's usefulness with clustered trash mobs. Has little ammo as backup (just 2 more grenades) so try to save it for moments of need, but it's a prime candidate to keep and use sporadically.

Molten Fists Melee
12 Crystals to upgrade
Two fists that punch repeatedly and has a DoT alt-fire that creates lava pools. As with all melee weapons, not a personal favorite of mine and mostly functions the same as your basic sword.

Magma Cannon Blue Ammo
12 Crystals to upgrade
A quasi-shotgun pistol which has no magazine. Since it just feeds off your general Blue Ammo reserves, you can keep firing without worrying about reloads. It fires in a cross-pattern which spreads out the further away, and the alt-fire just fires a more powerful version without spending ammo. A prime candidate to keep if you don't improve your Revolver.

Railbunker Red Ammo
12 Crystals to upgrade
A focused-target spear which shoots small explosions. Holds 4 spears in its chamber, and fires decently fast. Can be a good way to deal a lot of damage to dangerous enemies or bosses, but since Red Ammo is so scarce, you should consider if you want to upgrade this weapon or the Larva Launcher.

Deepgun Green Ammo
11 Crystals to upgrade
There's a lot to love in the Deepgun: a fast-firing gatling gun which fires continuously without the need to reload, feeding directly from your Green Ammo reserves. It has a jumpy kickback but you aren't using it to snipe faraway enemies. The icing on the cake is it's alt-fire: a barrage of explosions in front of you, from left to right, which is perfect for clearing clustered smaller enemies. A great weapon which imo supersedes the Bone Riddler.

Srynge Yellow Ammo
11 Crystals to upgrade
A poison flamethrower that continuously spews green flames. Alt-fire leaves an area of effect that deals damage over time. You'll find it competes with the Trench Gun in the close quarters department, which one to prioritize will come down mostly to personal preference.

Bone Sword Melee
11 Crystals to upgrade
A two-handed blade with a slower, three-hit combo when compared to your other melee weapons. Alt-fire is a forward lunge. Comes down to preference, but it was a no-brain melt for me.

Wormhorns Blue Ammo
14 Crystals to upgrade
A dual uzi organ-cannons that fire in a random spread. It's secondary fire is a lenghty animation that buffs your damage for a short time. Honestly, straight into the melting they went.

Tarot Cards Green Ammo
14 Crystals to upgrade
One of the most interesting weapons in the game, which I kept for their novelty. Throws tarot cards (upgrade fires 3 cards in horizontal spread) and has no magazine, which allows you to throw cards indefinetly. But the interesting bit is the 1-mana secondary fire, which draws a single card from the deck. Effects include damage, speed, mana regen, generating magma pools, throwing meteors or spawning enemies. It's fun but trying to draw the card you want in a pinch is unreliable (like mana regen), since there's at least a dozen possibilities.

Torment Heads Red Ammo
12 Crystals to upgrade
Another interesting choice, the Torment Heads are in the league of the Piledriver or Larva Launcher for high-damage / low-ammo types. This fires a delayed area of effect attack which deals high damage, albeit a bit slow for my liking. The alt fire resurrects enemies as allies for only 1 mana, which is an interesting effect, and after upgrading it, the reload consumes your health whilst refilling your mana to its max. Interesting but very situational. Can be a good tool if you use a lot of mana.

Shrike lance Melee
14 Crystals to upgrade
A spear-and-shield combo which you swap using R (from offence to blocking). Probably the most interesting melee weapon in the game, due to its mechanics. Holding the mouse 1 will charge the spear whilst blocking with the shield, release to stab at medium range. "Blocking" prevents projectiles and enemies from getting close, but you can get damaged in melee regardless. So a bit weird.
Again, up to personal preference, but I found it a bit clunky.

Astral Tome Blue Ammo
14 Crystals to upgrade
Cool-looking book and gizmo which fires projectiles and secondary AOE blasts. After some weird additions, this feels quite a normal weapon. Up to preference but I prefer the Revolver or Magma Cannon in this ammo type.

Chimes Mana (!)
14 Crystals to upgrade
The chimes are a weird weapon as in they use no ammo at all, only work on mana. They spawn minion pets which will fire at enemies for 1 mana. Upgraded version summons two minions.
The secondary fire stops time for 10 seconds for 6 mana. Quite insane and doesn't require you to upgrade it to use to its full effect. Just for the novelty and cool factor, I kept it to use sporadically. Can be nice pairing it with the Torment Heads to regenerate the mana you spend stopping time.

For those who want an example of a loadout after all the melting, this was my equipment in the later third of the game:

Bastard unupgraded
Revolver upgraded (Blue - Main weapon)
Larva Launcher upgraded (Red - upgrade not a priority until lategame)
Deepgun upgraded (Green - replaced Bone Riddler as soon as I got it from the Leviathan)
Syringe upgraded (Yellow - replaced the Trench Gun shotgun)
Tarot cards upgraded/unupgraded (Green - they are mainly used in their secondary Tarot Drawing effect for fun)
Chimes upgraded (No ammo - used mainly for stopping time, and occasional minion summoning)

I preferred to keep it simple and not having to swap constantly between weapon types. Hope that this is useful to you.