I know this is a little ...... pedantic, but the gunshot sound was clearly not the VD. I do pick up from the comments made that it implies she killed him.

I'm actually leaning towards that myself. If you look at the lore between them..... it's been said that the two of them held the same affection towards one another. That they complemented one another. Through reading the lore on variks, you find that she felt really guilty about locking uldren up in the PoE. Not to mention, right before the screen cut to black, even the guardian was starting to doubt themselves. Makes me think that the guardian couldn't do it at the end but that it was Petra who did based on the relationship she had prior to the event.

She also makes a little side comment when giving you the vestian dynasty, could be hinting at that being that gun that was used to end Uldren.

It's whoever you decide if was. Actually clever by bungie. If you wanted to cap him, you can say it was you. If you think you would never be a cold killer, Petra did it. Or cowardly writing. You decide!

Nah, the gun sound would have been different.

Listening to it, I don’t think it sounded like the Ace of spades

Well, it certainly didn't sound like the Vestian Dynasty. Even if it didn't sound like Ace, it did sound like a hand cannon. Petra's weapon is a side arm, an energy side arm at that, so it doesn't even have a 'crack' sound as a bullet is fired. There's no way it was her if we're comparing weapon sounds.

That echo was purposely put for dramatic affect and ambiguity. But you can faintly hear the sound of sidearm at the very beginning of said echo. [https://youtu.be/1-_kcB8xH8k][/url]

It's not about the echo... ANY weapon can echo, as an echo is simply the rehearing of rebounding sound. The point is that it did have the crack to it, and it did not have an energy sound to any part of the gun firing. Even if you want to argue it wasn't a HC or Side arm. You can't argue it sounds kientic and not energy like. Thus, the Hand Cannon is the only option.