These are the 4mm High Current Bullet Connectors from Castle Creations.
FEATURES: Gold plated Fits wire diameter up to 4mm Use 16 gauge to 13 gauge wires Up to 75A current
INCLUDES: Three sets of bullet connectors
REQUIRES: Solder and soldering iron - HCAR0776 and STAR2000
SPECS: Length: .66" (17mm) Diameter: .15" (4mm) inside (where the wire is soldered to) Diameter: .19" (5mm) outside