A conviction for armed robbery in Massachusetts requires the prosecution to prove two elements: (1) the defendant committed a robbery; and (2) the defendant was in possession of a weapon. The robbery can be committed in one of two ways: (1) by force and violence (i.e., the actual force prong) or (2) by assault and putting in fear. Under either prong, the degree of force is immaterial, so long as it is sufficient to obtain the property against the victim's will. It is not necessary that the victim be placed in fear. In addition, the defendant need not use the weapon during the commission of the robbery, nor need the victim be aware of the weapon.
Under these circumstances, the court determined that a conviction for armed robbery in Massachusetts does not meet the definition of a violent felony, which requires a statute include as an element the use, attempted use, or threatened use of physical force against the person of another. Because the degree of force is immaterial, any force, however slight, will satisfy this prong so long as the victim is aware of it. This does not meet the violent force requirement for a violent felony. Moreover, since the statute does not require the use or threatened use of the weapon, the fact that a defendant must be armed does not transform the conviction into a violent felony. Given the similarity in the definition of a violent felony for sentencing purposes and a crime of violence for immigration purposes, this decision could be excellent persuasive authority for arguing that a Massachusetts conviction for armed robbery is not a crime of violence.
The full text of US v. Parnell can be found here: http://cdn.ca9.uscourts.gov/datastore/opinions/2016/04/12/14-30208.pdf