Regulus ignored Potter, because it wasn't like they could be punished for testing the spells. Prisoners were supposed to attempt to escape. They were doing something to help. Instead he thought about the spells.
"Before we test it, though, let's think this through. We have some magical barrier that covers wall-to-wall floor-to-ceiling, or one would imagine so, because the alternative is just stupid," he said. "Now, if we do cast a tunnel, we have the walls and it's empty inside. The question is would the barrier still exist there or are you isolating completely? If it's the first, we get zapped. If it's the second, then we just walk through." He looked at the empty air again. "I suppose there is only one way to find out for sure. Let's do it."