According to legend, these devices were scheduled to first ship from New York to doughboys in Europe on what turned out to be Armistice Day.  
The "secret" to early batches of these devices was their licensed Blish lock.  US Marines were called in to protect mail routes after one of these devices was used on a mail carrier in the town of Elizabeth, New Jersey.  Although these devices had a much shorter range than the BAR, they only weighed 10 pounds so were much more portable.  This technology was developed at the newly founded (*) Auto-Ordinance Company.  According to legend, these devices were scheduled to first ship from New York to doughboys in Europe on what turned out to be Armistice Day.  Nicknames for these weapons included the "Chicago Typewriter" and the "Trench Sweeper".  Two of these highly-inaccurate guns found at Fred Burke's house were tested to be the same ones used against Bugs Moran's gang in the St. Valentine's Day Massacre.  For 10 points, name this type of automatic weapon used by Prohibition-era mobsters.
The "secret" to early batches of these devices was their licensed Blish lock.  
 tommy gun [or Thompson submachine gun or chopper; accept Trench Broom or Trench Sweeper or Chicago Typewriter or Chicago Piano before the "nicknames" clue; prompt on gun or rifle or machine gun or submachine gun]