After i posted that i did come across this on accurate shooter
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The case capacity to bore ratio is one of the two key metrics in internal ballistics affecting powder burn rate. (The other is bullet weight.) The usual method of determining the first is to divide case capacity as measured by the weight of water it holds in grains by the bore area in square inches. The lower the value of the result, the faster burning the powder; the reverse for higher numbers.
Using 56gn water capacity for all 308 Win variants (to reduce effort - they vary by a grain or two in practice according to calibre) you get:
243 Win ...... (0.046) ......... 1,196
260 Rem ...... (0.055) ........ 1,000
7mm-08 ...... (0.063) ......... 873
308 Win ...... (0.075) ......... 747
338 Fed ...... (0.090) ......... 622
358 Win ..... (0.100) ........ 560
The figures in brackets are the bore areas in square inches.
7mm-08 and 308 with ratios of ~ 873 and 747 respectively are far enough apart to justify different burn rate powders for optimal performance, although the 7mm-08's value is still low enough to still see good results with fast to mid-speed propellants like VarGet. (Its ratio is very close to 6.5X47mm Lapua's, another design that straddles the VarGet / 4350 powder watershed.)
At 747, 308 Win is a step lower, and counter-intuitively, is very close to that of 223 Rem - why they both find the same range of powders optimal usually.
The other factors are custom extended throats (which have the same effect as increasing case capacity through seating any given bullet less deeply in the case), and bullet weight. Because of the different bore sizes, 175gn 284 and 308 bullets aren't directly comparable. On a 'scaled' basis, a 175gn 7mm bullet is roughly equivalent to a 200gn 30-cal design.
I shoot a long-throat 7mm-08 in F-Class with the 160gn Sierra TMK normally. My usual powders are Lovex SO65 (Shooters World Long Rifle in the US) and Viht N160. Neither is a suitable powder for 308 Win. Powders suitable in 308 work in this set-up, but usually pressure-out at around 100 fps lower than the slower burners. Despite a throat long enough to see bullets loaded to just short of 3-inch COALs, I've tried and rejected 175s in the cartridge as requiring over-compressed charges and producing insufficient velocities. (You can use them in the cartridge, but they need still more freebore than my chamber has.)"
I'm trying to get the 176 a tip to around 2700-2750 fps so im looking at powder selection now. I have staball match on hand but i dpn't think it'll quite get me there. Might hunt for some 8208-xbr for that