From Zack Smith in the 6.8spc FAQ. Will a 6.8 in a 18" w/rifle length gas system work?
My guess is no. I believe Wes & Paul did some experiments with regard to gas port location and size.
I did some studying with QuickLoad:
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If I use QuickLoad to plot chamber pressure as function of bullet position in the barrel:
223, 61299 psi max, 3215fps from 18" bbl
approx 27500 psi at 9.5" (midlength port)
approx 20000 psi at 12.5 (rifle port)
223, 55123psi max, 3118fps from 18" bbl
approx 26000 psi at 9.5"
approx 19000 psi at 12.5"
6.8, 55069psi max, 2673 from 18" bbl
approx 19000 psi at 9.5"
approx 14000 psi at 12.5"
using different cartridge model datum
6.8, 55086psi max, 2640fps from 18" bbl
approx 18000 psi at 9.5"
approx 14000 psi at 12.5"
When I take 6.8 down to 26.5gr RL10X, the 9.5" pressure is more like 17500psi.
6.5 Grendel is closer to 223/556.
(This analysis assumes that the pressure is equalized throughout the combustion chamber, ie, pressure at bullet = pressure at breech, which is borne out within about 10% by Armalite's pressure graphs.)
So it looks like the 6.8 midlength port pressure is roughly equivalent to light 223 loads with a rifle-length gas port.
When you download 6.8, the port pressure is WAY less than even a rifle length 223 with moderate loads