You can buy the guns from chet in goodsprings or by getting the key from a king boatfly(i added a map marker near goodsprings) and using it to unlock a floor safe in chet's room.

Stats:

Dart Pistol
WT=1.5 , HP=500 , Value=5000 , DMG=75 , Fire Rate=0.7 , Clip=10 , Silent , DPS=234, Spread=0.5
Dart Shotgun
WT=4 , HP=300 , Value=7000 , DMG=150 , Projectiles=7 , Fire Rate=0.5 , Clip=8 , Silent , DPS=600 , Spread=50
Dart Sniper
WT=3 , HP=300 , Value=7000 , DMG=150 , Fire Rate=0.7 , Clip=6 , DPS=289 , Spread=0.015
Dart Minigun
WT=10 , HP=2000 , value=10000 , DMG=30 , Fire Rate=50 , Clip=240 , Projectiles=2 , DPS=1500 , Spread=5