This G-Force Can Make You Pass Out! | Lonely Planet’s Year of Adventures | BBC Earth Explore 

They weren't barrel rolls. A barrel roll is only 1G, it's one of the most gentle aerobatic manoeuvres you can do.

Vladimir was doing what is known as 'steep turns' with a roll thrown in during the turn. Vladimir could have done sustained 6 - 7 or even 8G steep turns if he had put his jet in a dive with throttle at about 90 - 95% settings.