Sorry -- Someone please tell me this isn't, or hasn't been, common knowledge for like -- YEARS....
Most Pros use the uninsulated butt connectors, they're readily available - and heat shrink it. I personally have about half a zillion feet of various sizes of shrink tube in all the colors so that you don't even see the splice.
I'm glad that others are discovering this because there's nothing uglier than a bunch of sore thumb looking splices...