10pc 6 AWG Wire Connectors, 80A Quick Terminal for Splicing and Wiring, Circuit Connector with Screws and Heat Shrink Tube Included

Get what paid for

Haven't used yet but looks like they can do the job well made.

I used these terminal connectors to extend some mains in the garage to install another socket. Although the set was provided with heat shrink tubing in the end I wasn't happy that it was safe enough and so ended up housing the joins in a box anyway - which means I might as well have used regular terminals.In summary, the joins are solid but, for me, it isn't as good as a more standard solution. This is a proper inline solution and so if that's what you need then maybe it might be right for you.

Solid metal tubes with screw holes and small screws that are not regular in design, being hollowed out such that the supplied key tool fits in them for tightening them into the holes. Each joiner comes with some heat shrink tubes too, in case the join is to be made permanent, otherwise you'd have exposed metal. The tubes are quite wide so make these better suited to larger cables as an alternative to other junction boxes or soldering, though I would consider adding more insulation to the outside of the joins to more closely match what you would get with plastic cased ones.