The double tube system for disco, prod, and mrod was developed by Lloyd Sykes. Norm, the original owner of Discos R US, inletted the stocks used on them.
When Lloyd initially stopped making them, someone else was suppose to pick them up, but never really materialized.
The Prod version came in aluminum or steel. I have one of the steel ones, and also made one of my own from another prod tube. They provide double the shot count at the same Es. One consideration though, it does not allow for intube regulators. There is a little more maintenance and assembly / dis-assemby work. I found them much more balanced and less bulky than a bottle gun.
Looks better too, Mr. John!
I don't like the looks of bottle guns (never will), although I do have one that is still unfired and unaired.
Almost exactly a year late on this one, ha ha.
Fuzzy Grub has the story straight, thank you. Design, engineering, and manufacturing are all solid, as was the destructive testing. The aluminum tube version doubled the air capacity and added only 7 ounces to the pistol. The only drawback was the addition of a few more o-rings and the associated opportunity for air leaks. Rather minor.
Red-neck engineer. Funny. Humor disguised as an insult. It's advisable to have at least a minimal understanding of something before passing judgement. Still plenty of 10 year old airgun technology holding its own in the field.