Ok let´s say that I have Endless Whispers and Bronze Bombshell on the battlefield, then I Donate the bomb to my opponent, he must sacrifice it and when is put into my graveyard the effect is triggered and, at the end of the turn, it will be return under his control, right?

Now the question is: he must sacrifice it again right? if he does then the Bronze Bombshell is put into my graveyard and again the effect is triggered but the beggining of the end of this turn is already over, so when does the Bronze Bombshell returns?

Your first premise is wrong. Your opponent is the one who chooses where the Bronze Bombshell goes. In theory, they could choose for it to go to you so that it won't have to be sacrificed again. However, in a multiplayer game, they could just as easily give it to yet another opponent, who would then have to sacrifice it and get to choose where it goes. The trick to keeping this damage hot-potato going is having a simple sac outlet in play so you can pass it around some more if it comes back to you at any point.

I'm going to use AzureShade's example to answer your "end of turn" question. Endless Whispers sets up a delayed trigger to return the creature to the battlefield at the beginning of the next end step. Let's say you're in a three player game with you, Opponent A, and Opponent B.

1) You give Bronze Bombshell to Opponent A during your turn.
2) Bronze Bombshell triggers, causing Opponent A to sacrifice it.
3) This triggers Endless Whispers, setting up a delayed trigger.
4) At the beginning of your end step, the delayed trigger triggers, and Opponent A decides to give the Bombshell to Opponent B.
5) Bronze Bombshell triggers, causing Opponent B to sacrifice it.
6) Endless Whispers triggers, setting up a delayed trigger for the beginning of the next end step. Since you're already in the middle of an end step, the delayed trigger won't go off until the next turn.
7) Your turn ends and Opponent A's turn begins. At the beginning of that turn's end step, Bronze Bombshell will return again.

Right, sorry I wasn't clear, but there is no way to creature a loop with this card to insta-kill everyone who is not you. It's more of a slow fun game of hot potato that you can't lose.

Great! thanks for your answers it's pretty clear to me right now and it's slow but it's commander so it's ok to be slow

I would like to point out that the player chooses a "target opponent" when the ability is put on the stack after the creature dies (CommanderJim's step 3). Not at the beginning of the end step (CommanderJim's step 4). Relevant if somebody has something like a Leyline of Sanctity out when the creature dies but no longer has it when the end step begins (cause it got destroyed or somebody gained control of it or something).