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MrBeast 's standing by his choice to trap a stuntman inside a burning building for a cash prize ... arguing the guy's a pro and firefighters were on standby ... so, it was all totally safe.

Here's the deal ... the YouTube star recently posted a video which pushed a stuntman to the absolute limit -- firing him out of a cannon, surviving massive explosions ... and even getting tied up in a house before it's lit on fire.

It's this last stunt many people online popped off about ... and, if you watch the clip, it's not hard to see why.

The man escapes from a series of ropes before grabbing a couple duffel bags, breaking a glass door and dropping them outside.

Instead of running from the fire, the stuntman heads back into the building ... grabbing more bags and repeating t

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