[MMMMMM]

Nutdawgz,

If the burglar couldn't get out through the overhead door--and apparently he couldn't--he might have been forced to exit through the house, precipitating another encounter with Dunn anyway.

Dunn likely realized this and grabbed his gun to be sure he could defend himself. His going back into the garage was pre-emptive in nature, and at that point the burglar had already extricated himself, and proceeeded to attack Dunn again. Better that Dunn handled it this way (instead of waiting another 30 seconds perhaps) for the burglar to realize the only path of escape was through the house, whereupon he might have come charging through attacking Dunn again and perhaps catching him off guard.