If you take a .729" bore & just figure 25" ahead of the crimp, you have 6oz of water in the barrel. For a .308" bore it would take 23" ahead of the bullet to hold 1oz of water. As has been stated the rifle bbl walls are much thicker than the shot bbls. It is much more common to see burst shotgun bbls than rifle bbls. Quite often a rifle bbl with an obstruction will bulge but not actually burst. In this case though it is more a thing of having an excessive weight to move than an actual obstruction I would think. Julian Hatcher reported on a .30-06 03-A3 service rifle using a 152gr M2 service round @ 2800 fps, being placed in a tank of water, ensuring the barrel was filled & fired by remote control. There was only about 6" from the muzzle to the tank wall. Upon firing the bullet the bullet penertrated the 6"of water, 3/64" of steel tank wall, 4 7/8" soft pine planks placed outside the tank & then buried about 2" into a solid oak plank. The bolt had to be opened by tapping with a rawhide hammer. The case head behind the cannelure had expanded by .032", the primer pocket was expanded from .209" to .228", primer was pierced & black smoke observed escaping the water's surface near the breech. Though this gun was not equipped for taking pressures as the expansion was greater than that of a 70K psi test cartridge it was estimated pressures ran higher than 70K. It was noted there was about 385 grains of water in the bbl. There was no bulging of the bbl or other noted damage however, a testimony to the strength of the 03-A3. "NO" recomendations were given for firing a gun underwater, & that's "No Myth".
