Wouldn't it be 0 inches? All the barrel does is slow it down, and the barrel certainly doesn't make it gain speed. So as soon as the explosion propels the projectile, wouldn't it be at maximum velocity?
A slow burning powder will continue (and does)to build up pressure in the barrel. A lot of variables. They were building a super gun a few years back with a barrel length that was in I bel sections and shipped. It would reach Israel from Iraq.
In documents filed with the UN Special Commission 18 July 1991, Iraq admitted possessing a gun with a barrel 350 millimeters wide and 45 meters long and that it was building a second one. The commission noted that the gun would have been inaccurate for conventional armaments, and that it was trying to determine if the weapon was intended for chemical, biological, or nuclear use. The superguns were potentially capable of firing chemical, biological and nuclear weapons to a range of up to 1,000 km.
The high-ranking Iraqi defector Gen. Hussein Kamel al-Majeed said Iraq was working on a space weapon launched from the supergun.
"It was meant for long-range attack and also to blind spy satellites. Our scientists were seriously working on that. It was designed to explode a shell in space that would have sprayed a sticky material on the satellite and blinded it."
He also said the supergun could have delivered a nuclear device.
Following the Gulf War UN teams destroyed one 350 mm. supergun, components of a 1000 mm. supergun, and supergun propellant.
Iraq used the petrochemical complex two (PC-2) project as a front to purchase components for Gerald Bull's super gun. Matrix Churchill was a long established Midlands based machine tool manufacturer which was purchased in 1987 by an Iraqi controlled company, TMG Engineering Ltd, which was in turn controlled by another Iraqi controlled company, Technology and Development Group Ltd (TDG).
The military uses of Matrix-Churchill machines are the prime reason Iraq was interested in purchasing the company. Acquiring Matrix-Churchill gave Iraq access, not only to the machine tools, but also the computer programming, tooling, and other components needed to make a wide variety of munitions as well as other applications in aerospace and nuclear industries.
The Iraqi NASSR Establishment for Mechanical Industries contracted with the company for the supply of machine tools for a project, code named "ABA", to manufacture parts for multi launcher rocket systems.
In addition, supergun components were fabricated in separate parts by factories in England, Spain, Holland and Switzerland. Acting on an anonymous tip, British Customs seized the final eight sections of the Super Gun in November 1990.
The work skirted the law but remained legal, as illustrated by Britain's unsuccessful prosecution of the case, following the joint British-American sting operation that uncovered key supergun equipment transfers.
Trappy, in that the conversion of potential energy to kinetic energy is not instantaneous the barrel is needed to contain the explosive force (I know, it's not technically an explosion...) so the energy may be transferred to the projectile.
The expanding gas cloud from the burning powder serves to push the projectile forward, as it does so there is a conversion and transfer of this potential energy to the kinetic energy of the projectile (physics 101).
There will be an optimal length for this for each cartridge. At some point friction will start to overcome this and velocity will start to drop.
Historically this length tends to be in the 20+ inch range for most cartridges (this is why rifle barrels tend to be in the 22 ~ 24 inch range). If the barrel is shorter than this optimal length velocity will be decreased (typically this is about 100 fps per inch the barrel is beneath the optimal length).
I do not know the loss in velocity to friction once you exceed the optimal length.
My Savage with, I believe, an 18" barrel chronographs 2530 fps at six feet from the muzzle (variation of +/- 30 fps or so) while my Taurus Tracker hits 1950 six feet out. I know that there is some loss due to cylinder/barrel gap, but a friend's 10" Contender only gets 100 fps more than the Taurus.
Maybe I should take the barrel off the Tracker so I can get maximum velocity. Can't do that with the rifle as it would then be illegal. Gee, Trappy got me thinking.................not.