double wall shrink tube has adhesive on the inside like hot glue. helps to seal the joint.
http://info.waytekwire.com/blog/heat...protection-101
Dual wall heat shrink tubing protects against moisture and corrosive environments, while providing electrical insulation and mechanical protection using engineered adhesives. Dual wall tubing products consist of a crosslinked outer jacket and an inner layer of adhesive or encapsulant. During installation, the adhesive or encapsulant lining melts and flows, creating a moisture-resistant protective barrier. Benefits of this type of tubing include controlled amounts of adhesive and consistent results; compliance with AMS-DTL-23053 standards; UL recognition/CSA certification; and RoHS Compliant.
I copy/pasted that, can you tell, lol.
some butt connectors have solder inside and also shrink themselves with the heat applied. home depot even sells them
https://www.homedepot.com/p/Tyco-Ele...3-12/202204351