Got this from McMaster-Carr. Three feet of 4130 Cro-Moly for $18.
The hardest part of the project--for me--will be threading the tube for the gas tube end cap at the muzzle end.
I had a really awful M22 x 1mm die that I bought for super cheap--it was about $8--shipped. When it arrived, it wasn't the nice looking split die pictured in the listing. I sent the seller a few helpful listing tips without mentioning words like "mail fraud". At least I got positive buyer feedback. No extra charge for the tetanus.
Lacking a tailstock die holder and not being interested in setting up the lathe gears for metric threading, I turned the end of the tube to be a snug slip fit into the die threads then cut a taper to start the thread. Idea was to let the tube center itself in the die. Aluminum sheet pads the steel tube in the chuck.
Die slips over the turned down section of tube before it engages the taper.
I didn't have a proper die stock for this thing. The die has an OD of 45mm. The machine tool supply place I visited a mile from the house had never seen or heard of a die with a metric OD. SO, that meant no dice on a handle and I wasn't about to spend half the day making one. That's OK, I wasn't exactly enthused with spending more money on the mail fraud die. Improvised (barely) with a strap and pipe wrench.
But it was enough. Here, the guide has been cut off and faced leaving just enough full size thread for my end cap.
Astonished that the die worked this well.
Test fit: The bulk fitting from the QB fits into the tube and the thread locks down tight. Nice surprise --the bulk CO2 fill cap didn't have to be turned down to fit into the tube.
Now it's a matter of cutting the tube to length. This is a high precision tape measure kinda cut. 27" with the bulk fill adapter installed.
The Ultra Fine Point Sharpie makes the mark.
Cut the tube, faced and beveled the end.
Now, I need to disassemble the QB77 down to a bare gas tube and start transferring hole and slot locations.
More in a couple days. I've got a gun to strip.