I could accept that the powder is ignited within the first 6" but that is about as far as I could go on that. Otherwise, the logic would be a Ruger 44 Carbine would have the same muzzle velocity as 6" barreled pistol in that cartridge. Or do I misunderstand?

I have also read articles supporting 28" barrels as in driving modern cartridges to full velocity. But it has been awhile so I will have to think on where they are.