Yes, gun was built around a design intended for use in a fight, so both buck and slug would be the norm. So not only better accuracy, but the fixed barrel eliminates any chance of the barrel getting loose under those conditions, which would increase headspace and risk having the gun not fire a live round because the primer strike would then be light.
Does not occur often, but it has on interchangeable barrel LE and military guns. Just saw a video of this recently on a military shotgun being tested.
Flip side, many civilians complained about the fixed barrels for greater difficulty to clean and the inability to swap barrels around. The current gun is marketed for civilians, so they got rid of the fixed barrel, which also saved money, as the fixed barrel is more expensive to produce.
