most people shoot across course with 6xc at least what i have read here. i shoot 100 300 500 ground hog shoots. need tighter group at 100yd or might say a more reliable group at 100.
Don't assume Across-the-Course shooting has lesser requirements in either accuracy or precision. (See
http://celebrating200years.noaa.gov/magazine/tct/accuracy_vs_precision.html
for a visual of both.) When firing from position, it is a given there is more "noise" affecting both. Still, 6xc is competetive. For example, in 300 Meter International shooting (different form of position shooting), 6xc is making inroads - it is the cartridge for the Norwegean national team, and possibly another (Sweden?), and has won at least two team championships. Significance? Since the 1970s, the dominant cartridge in 300 Meter has been the 6BR.
Having said the above, here's a link to the load data used by David Tubb, developed using REMINGTON brass, Sierra bullets (moly coated), and a Schneider polygonal rifled barrel. (Brass, bullets (and coatings) and barrels are noted, as all have an effect on pressure.)
http://www.davidtubb.com/tcom_images/6xc_images/6xc_dies_six.html
NOTE: Powders of choice are N150/Varget and slower, with maximum velocities achieved using H-4350 or N-160.
From experience, 70 grain bullets can safely be fired using charges intended for 107s. Whether they're precise enough for your application is for you to decide.
As a thought, you could always buy a few boxes of Norma 6XC ammo, and use them for benchmark loads. IIRC, they may be loaded using Norma 203B powder (similar to Reloder-15, if not the same powder.) Factory ammo which shoots 60 shot groups of just over 2" @ 300 meters can't be that bad.
