Recently purchased a CMP Hbar 5.56 barrel and an upper that had a barrel chambered for a 6x45mm... what exactly is this round and where can it be found??

The 6x45mm is the .223 Remington necked up to 6mm with no other changes to the original case. This is a wildcat cartridge so you will need to hand load ammo for it. It is a easy cartridge to reload and dies are not that expensive. This cartridge was first developed around 1965 by bench rest shooters it is know to be very accurate and set many match records in its day.

Joe

Edit to add:
This cartridge is also known as the 6mm/223.

Great help, thanks!!