What is this based on? It isn't the same as the 20 tactical is it?

It's the .17 Rem. necked up to .204 caliber. Similar to the .20 Tac, but has a longer neck and slightly different shoulder angle, I believe. Here's a link to an article online about the .20 TNT: http://www.smallcaliber.com/wc-20tnt.html

Here's one to the .20 Tac: http://www.smallcaliber.com/html/tactical20.html

Hope this helps.