From Reason:

The magazine ban attempts to divorce today’s common arms of law-abiding citizens from today’s common arms of law enforcement officers, including sheriffs and their deputies. The divorce, contrary to the wishes of both parties, endangers citizens and officers alike.

The arms of ordinary law enforcement officers are carefully selected for only one purpose: lawful defense of innocents in civil society. Throughout American history, many citizens have looked to law enforcement for guidance in choosing arms for the same purpose. Denying those arms to citizens and to retired law enforcement officers  endangers them for the same reasons that denying these arms to active law enforcement officers would endanger them. The most important reason is the necessity of reserve capacity, as detailed in Part II.