Greg,
Thanks for the video! I had never seen the guns in the operating position on a Dauntless and never realized they had a pair of doors aft of the cockpit that opened to let them come up. The pair of guns on this aircraft are definitely .30 cals. I base this on the overall size of the guns and the rounds going into the feed chute. The .50 cal Browning gun is huge (overall length 65 inches) and the BMG round is 5.45 inches long and the case .80 inches in diameter. The rounds in the vid don't look anywhere near that big.
Some of the early tanks I trained on still had a .30 cal model 1919 Browning coaxial gun in the turret. So, I've fired those on quite a few occasions as well.