I've been watching hoping to see someones results with a sub 18" creedmore or 260. I suspect, the popular powders need the 20"+ of barrel because of how slow they burn. If you decided to go with a creedmore I'll be following to see what velocities you get.
If I may I'd like to offer a suggestion, as long as you're in the experimental mind frame. In testing alternate cartridges for the military the AMU had taken a Grendel(6.5mmx39) and with all else being the same did some testing with a 6.5mmx45 based on the Carcano case. The carcano I believe can utilize a .308 bolt. Someone correct me if I'm wrong please.
This might be worth looking into. I'd suspect this cartridge would work well with many of the powders that work for the Grendel and more that work for the .308 that the Grendel doesn't have the case capacity to utilize.
Thanks for the suggestions.
Grossly simplifying the very complex science of internal ballistics, one could say that the burn rate of the powder has to match the volume increase of the space behind the bullet as the bullet moves down the barrel. Shotgun shells for example need very fast powders because the volume for the burning powder has already doubled by the time the wad leaves the shell. With a too slow burning powder, the pressure will drop quickly and the powder will eventually stop burning.
If OTOH we want to burn a large amount of powder behind a small diameter bullet (= slow volume increase), we need a slow powder; otherwise the pressure would get out of hand.
Looking at how quickly the volume behind the bullet grows, it should not surprise that the .22 Hornet, the .44 Magnum and the 300AAC are all very happy with powders like H110 or Lilgun. While the cartridges have vastly different bullet diameters and weights, their relative volume expansion rates are in the same ballpark.
The Grendel can and should use faster burning powders among the 6.5mm offerings because the case volume is much smaller than a 6.5CM or similar cartridges. Therefore, the relative volume increase of the Grendel will be larger as the bullet travels down the bore and we need to make up for this with a faster burn rate.
Obviously, shoulder angle and other case dimensions play a role as well as the weight/hardness of the bullet, energy density of the powder and other details, but roughly speaking, large case volume with any given bullet diameter and weight requires slower powders than small case volume with the same bullet diameter/weight.
If we would design a new case that uses a similar powder volume to shoot the same bullets as the 6.5CM, we would end up with a very similar powder burn rate and would need a very similar barrel length to get to the same muzzle velocity.
Hope that makes sense.
