Here are the two company's currently making tube gun kits
“The Original Tube Gun”
COMPETITION SHOOTING STUFF
A tube gun article on 6mm.com
Eliseo R5 6mmBR Repeater Target Rifle Tube Gun
Using a Rem 700 receiver..... there appear to be some aftermarket receiver's you can use .
I'm in the early stages of gathering parts.
You can glue the receiver in the tube or use a special round recoil lug.
Do searches on the 6mm website,they're very popular in some shooting disciplines.
I'm going to use the MAK kit because it allows the use of AR15 stocks.
Hope this gets you started in the right direction,there is info out there its just scattered around.