Both Truth and the Unfriendly Giant with the Tripod mod (3 in the chamber) allow to carry six rounds, but it doesn't explain why the Valedictorian gets 1-shell pickups for a total of 4, and the other Unfriendly GIant gets 2-shell pickups for a total of 5, even though both are stated to have a magazine of 2.

Confusing!

Even more confusing, is the fact that before Truth was upgrades to 3 rockets in each mag, I could have 8 Max (6 without heavy ammo perk), which is the same as before.

This may not be correct, but the theory holds well enough that I haven't noticed it not holding :)

A ammo pickup will be 50% of max capacity rounded down. So,
Max 4 = 2 shells
Max 5 = 2 shells
Max 6 = 3 shells.
Max 7 = 3 shells.
Max 8 = 4 shells.

Gjallarhorn carries 7, but when I play as my Titan, each ammo drop gives me four shells.
Playing as my Warlock, each one gives me two shells... Both have gear that improves ammo capacity.

*shrugs*

A ammo pickup will be 50% of max capacity rounded down. So,
Max 4 = 2 shells
Max 5 = 2 shells
Max 6 = 3 shells.
Max 7 = 3 shells.
Max 8 = 4 shells.

This is why I carry Truth into PvP combat with armor that gives me 8 shells max, I always come away from ammo drops with 4 shots, and the ammo drop bug has no consequence in PvP.

In PvE I haven't paid really close attention to heavy ammo and whether that principle breaks down. Lately with all the Arc enemies I've been using machine guns instead of Gjallarhorn in PvE. This is where it becomes even more apparent that a single ammo pack gives you 50% max. I think it's the rounding down with really small numbers that gets people.

you can carry 6 shells if you have armor that gives you more capacity. of course, you lose ammo if you die.......

...what she said.

The Rocket launcher from Crota's End holds two shells per mag, 4 rounds MAX.

you can carry 6 shells if you have armor that gives you more capacity. of course, you lose ammo if you die.......

Confusing!

But then, I have a launcher with field scout, holds 2 in the tube and 1 reserve when I pickup, so many not :/

I play a Hunter with no modifiers on heavy ammunition, I'm not sure that even exists.

Sometimes, I can carry up to six shells total, sometimes five, sometimes four.
Depending on the rocket launcher I have equipped, I will get either one shell per purple ammo pickup or in some cases two. It's not random, it's rather tied to specific rocket launchers

For reference, here's my experience.

Truth (upgraded version):
Magazine: 2
Two shells per pickup, max six

Unfriendly Giant (pre DLC)
Magazine: 2
No relevant mods (Surplus / Who's Next)
Two shells per pickup, max five

Unfriendly Giant (pre DLC)
Magazine: 2
Tripod (Tripod / Grenadier)
Two shells per pickup, max six

Valedictorian 9-44
Magazine: 2
No relevant mods (Grenades & Horseshoes / Clown Cartridge)
One shell per pickup, max four

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In spite of the potential 331 AP, this makes the Valecdictorian less attractive than an old Unfriendly Giant. Imagine you are out of ammo, and three purple ammo pickups drop: this translates to six shots from the Unfriendly Giant with Tripod, but only three with the Valedictorian.

What is your experience with this behavior? Is this tied to specific Rocket launchers?

Thanks!