I think you're over thinking this.
What exactly is a "device" as defined in the ATF rule pertaining to a silencer?
Already given.
Does a device necessarily have to be a separate part removable from the gun itself, or could it also be an integral part of the gun itself?
No. That require isnt needed to fit the definition.
Previous posters already stated that the silenced ammo through less power is not included, so what makes it not a possible device as per definition in the regulation when other gun parts seem to be included as devices?
Ammo isnt part of the gun. Just as gas isnt part of a car motor. Ammo has its own regulations such as not armor piercing.
How about if the device is not even attached to the gun but nevertheless intended to muffle the report of the gun like shooting an outside target from inside a sound-proofed box enclosing both the shooter and the gun?
Again, its not part of the gun... modification of the gun, etc. Thats a building; such as an indoor shooting range. Also, remember the ddfinition mentions 'portable firearm'. The building just confined the gun. Building a portable building such as a sound proof pope-mobile isnt practical.
Is it not true that for a particular given ammo a relatively louder report could be expected if fired in a shorter barrel than in comparatively longer barrel?
The length of the barrel is a design of the gun (such as rifle or hand gun) and is NOT designed to muffle the sound. If the barrel is designed to muffle the sound, and there are those barrels out there, it would fit the definition and are regulated because of that.
Based on the law, provided a "device" could also be an integral design or a design on a removable part for a gun, what would prevent a prosecutor from saying that the longer barrel installed in a gun is an attempt to "silence" the gun, however minimal, thus legally restricted without a BATF permit?
Mfg's submit their product to the Govt for approval - such as the Taurus Judge, and Mares leg. Or they base it of of case law. Thats what prevents a prosecutor from doing that.
After all, the amount of silence or reduction of the sound intensity is a relative thing but the law does not seem to give a clear quantitative base measurement on the amount of the reduction in loudness of the gun report in order for it to be considered not "silenced" enough as opposed to being illegally muffled.
Right. From their prospective, they dont want the sound muffled... period. In their mind,
its not about how much its muffled, its about they dont want people creating "devices" that muffle the sound.... period.