Barreling Attack — ruling 1

Upon resolution, this gives the targeted creature trample and creates a delayed-triggered ability that will trigger when the creature becomes blocked. The creature will get +1/+1 for each creature blocking it as that ability resolves. This means that if Barreling Attack is cast after blockers are declared, the targeted creature will gain trample but won’t be able to get +1/+1 because it’s too late for the ability to trigger (unless there is another combat phase in the turn).

Barreling Attack — ruling 2

Both the +1/+1 and trample last only until end of turn.

Barreling Attack : règles, fonctionnement et interactions (MTG)

Barreling Attack is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Target creature gains trample until end of turn. When that creature becomes blocked this turn, it gets +1/+1 until end of turn for each creature blocking it.

How it works : Barreling Attack

Original English text — Wizards of the Coast / Scryfall

#1May 1, 2012· Wizards of the Coast
Upon resolution, this gives the targeted creature trample and creates a delayed-triggered ability that will trigger when the creature becomes blocked. The creature will get +1/+1 for each creature blocking it as that ability resolves. This means that if Barreling Attack is cast after blockers are declared, the targeted creature will gain trample but won’t be able to get +1/+1 because it’s too late for the ability to trigger (unless there is another combat phase in the turn).
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
Both the +1/+1 and trample last only until end of turn.
No other card is named in this ruling.