welding a cord to form continuous loop?

If it's a nylon cord (you're not clear about just what it is) you can melt the ends and join the molten pieces together under pressure but the bond will not take much strain. A glue gun would probably give a less reliable joint.

Rope style cord would join best with splicing...
http://en.wikipedia.org/wiki/Rope_splicing

You might consider wire rope (Bowden Cable inner core) joined with a crimped ferrule - this a picture of a very big one ...

http://en.wikipedia.org/wiki/File:Wire_rope_with_thimble_and_ferrule.jpg

And smaller ones

http://www.tecni-cable.co.uk/Products/Double-Copper-Ferrules-for-Miniature-Wire-Ropes

Peccavi
May 2013

It depends on what you realy want/need to do.

If it's for a shrink sleeve, your glue gun may be hot enough. But, as the heat will be applied only to a spot, you'll need to slide it back and forth all around to get an uniform shrinkage. It will take much longer.

If it's to melt some solder strip, it's possibly not hot enough, but you can try.

Electro
May 2013