I saw that episode. I recall something different: rifle rounds penetrated the target all the way at the bottom of the water column, and a shotgun slug shattered the tank through transmitted shock.
They only tested the handguns and shotgun in the water column. Recall that all kept 100% of their mass, which gave them deeper penetration.
They then went to an indoor pool and fired the rifles.
Every round, including the 30.06 from the M1 Garand, fragmented upon impact with the water.