I found a fellow that has some of these barrels for sale. They are new, made by Olympic Arms back in the 70's for the military, and I am told they were going to go to the 6mmx45 but the war ended so they stopped the change, and these were leftover.

I am told it has the similar ballistic values to the 5.56, but with a third more power, because of the heavier bullet, 55 gr vs. 75gr. Kimber made/makes a rifle in this caliber.

Does anyone have any information on these barrels? Is the chamber SAMMI or NATO spec? Will the standard AR15 bolt work? Since it is a bit more powerful will I need a stiffer spring? Will modifications be required to the mag? I would like to build a new rifle based on this barrel but don't really know where to start.

The military was never going to the 6X45, tho some of the early designs use a 5.56 case as the starting point. The military was dabbling with the 6mm SAW, a totally different round. Since the round was never "NATO spec" it's doubtful the chamber would be.

As for the rest of the story given by this fellow, I don't think so.

All that is required to convert a 5.56 to a 6X45 is a new barrel.

I have a 6X45 barrel I purchased from the vendor known as "Old Sarge" back in the early 80s - in fact the Pacific Dies I have are labled "6X45mm Old Sarge".

I picked up this barrel for $12 when they were clearancing them, along with a thousand rounds of ammo and dies, I suspect it might be an Oly Arms barrel.  They made fairly good barrels if you are not wanting some sort of match grade accuracy.

It is an interesting cartridge but not much of a step up from the 5.56 with all of the heavy bullets made for it now.  

I have long wanted someone to go up to 6.5 or 7mm (and toyed with the idea of a 6.5 TCU myself).

Now that the 6.5 Grendel and the 6.8 SPC are out, the 6X45 would seem to be of limited value.  I think I did get an 85 BTSP to about 2750 with a 20" barrel.

Good luck!
Jim

As a comparison the XM732 6mm SAW round throws a 105 grain FMJ-BT at 2,520.

riposte,
How is the barrel marked?