The 9mm submachine gun is a small, yet capable weapon, able to empty a full magazine in under 3 seconds. The 9mm rounds are common and lightweight, making it even easier to spray-and-pray.
With the light bolt modification, this weapon deals 200 damage-per-second. However, it shares the common weakness of most automatic weapons in Fallout: New Vegas, in that much of its theoretical power is negated by the Damage Threshold system. In addition to losing much of its damage to the target's damage threshold, the high spread of the SMG requires the Courier to be at close range for all of the rounds fired to impact on the target.
The 9mm submachine gun can fire a total of about 2,745 standard rounds, the equivalent of 92 reloads (or 46 reloads when modified with drum magazines) from full condition before breaking.
Lucky 38 - a fully repaired one can be found in the basement next to an ammunition box on a table. (The player character can only access the Lucky 38's basement when taken there by Mr. House during The House Always Wins, I or by console commands.)
As the 9mm SMG is a one-handed weapon, it can be equipped without reducing movement speed.
The 9mm SMG shares its firing sound with the 9mm pistol.
Behind the scenes
The 9mm SMG is based on the M3 submachine gun also known as a "Grease Gun," albeit with a shorter barrel than its real-life counterpart.
PCXbox 360 Sometimes when switching to the 9mm submachine gun the magazine will appear forwards of the barrel with the rest of its orientation correct. Upon reload, the magazine position issue will correct itself. It appears to only affect the gun when the drum magazine mod is equipped. [verified]
9mm submachine gun with the drum magazine modification
9mm submachine gun as depicted in the Run N Gun perk image
“We have a little 9mm submachine gun, it looks like a World War II grease gun, but sort of compacted. And that's a really cool distinctive little weapon.”— J.E.Sawyer, Fallout: New Vegas video developer diary #2