Complex sentence: The entry shotgun or breaching shotgun is a further modified riot shotgun with an extremely short barrel and often only a pistol grip rather than a buttstock, and often a folding or collapsing buttstock if it is provided with one.
Split into simpler sentences: The entry shotgun or breaching shotgun is a role that can be filled by a standard riot shotgun, or one further modified for these purposes. They may have an extremely short barrel and often only a pistol grip rather than a buttstock, or a folding or collapsing buttstock if it is provided with one.