Thanks for the pics, clearly that is a Mohawk 600!
And that appears to be a Remington barrel. So my idea #5 is toast.
Still not certain if that is the ORIGINAL Remington barrel...(probably is..)
I was unaware that Remington produced any Mohawks in 6mm, although one thing I have learned is that in firearms, exceptions are often the rule. I will do some more research.
After dropping the model 600, (and 660) as front line items, Remington continued to produce the 600 as the Mohawk 600 for a couple years, and if I recall correctly only listed them in .243 and .308Win. I have a .243, have seen others, have never seen a Mohawk .308, but that doesn't mean they never built any...
One of the things that sometimes happens is that, when a model is nearing the end of its production, gunmakers will use up as many of the existing parts as possible, which leads to variations that were not catalogued items, and may even be one of a kind rarities. If I cannot find any listing of a Mohawk in 6mm it might be that your rifle was simply assembled with a late production Model 600 6mm barrel that was "left over".
As to the whole "Japan designed the 7.7mm to be able to shoot .30-06" thing, its just another of the BS rumors that float about in our military, and never seem to go away. Unless you are superhero strong, you CANNOT close an Arisaka bolt on a .30-06 round BY HAND. And I'm not sure a "bootheel to the bolt handle" would do it, either, though a hammer to the bolt (or a hydraulic press) would, eventually...
I've made a number of 7.7mm cases from 06 brass, and know a bit about the dimensions. An 06 case will NOT fit in a 7.7mm chamber.
The reverse, on the other hand, does work. You CAN chamber a 7.7mm and fire it in a .30-06 chamber!!!!
One possible point of confusion that either led to, or has kept that particular BS rumor alive is that a number of Arisakas had their chambers cut to .30-06, AFTER THE WAR, in the US, by civilian gunsmiths. NEVER by the Japanese makers during the war.
I have heard the "they can shoot ours but we can't shoot theirs" rumors about a number of things over the years, and with one exception (that I know of) all have been pure BS.
The only case I have ever found where the "they can shoot ours but we can't shoot theirs" MIGHT be true is the Soviet 82mm mortar. In WWII, virtually everyone's "mid bore" mortars were 81mm-ish copies in whole or part of a pre-war French design. US, German, French, (not sure about British, but likely) were all 81mm. For what ever reason, the Soviets made theirs 82mm,
possibly to be able to shoot 81mm ammo,
probably just as a convenience for their manufacturing process.
NO ONE has ever made,or at least admitted to deliberately making, their guns to take the enemy's ammo, and their own.
I have also heard the stories about how the Japs used "bamboo bullets", and stand ready to debunk that one, as well.