There is a magazine restriction of 10 rounds in Massachusetts. Magazines that hold more than 10 rounds are grandfathered in, as long as they were owned before 09/13/1994, and the owner has a LTC-A.
Some states have a limit on the number of rounds a magazine can hold. In these states purchasing, selling, or transferring a magazine of larger size than the law allows is a crime. In some cases magazines that were bought before the law was put in place must be registered with the police, in others they may only be loaded to the number of rounds legal in a state. The law may vary in some cities.