Thanks, the top board is a piece of 1x4 that is planed down and tapered slightly from top to bottom thin to thick respectfully with holes drilled every 2" or 50mm to receive the stud bolts. The bottom board is just a piece of 2x2 that is planed a little. Both are screwed into the side of the book case. The idea is to kick the bottom out enough so that the barrels rest in place without a tendency to fall out of place.
For thick woods I would use the 8x57. It is a wonderful cartridge especially for the handloader but admittedly I have a thing for anything 8mm. My second choice would be the 9.3x62, another sweet cartridge. For pronghorn I would use the 6XC. I have shot all of my pronghorn except two with a 6mm Remington and it has never disappointed me. The 6XC that I have is very accurate. The .257Wby would be another excellent choice for pronghorn or deer. I shot a friend of mine's Weatherby Lazer Mark .257Wby yesterday while zeroing his new scope. It is really nice and fast. We chronograph'd Weatherby factory 100gn ammo at 3475 - 3500 f/s. It has me thinking... a dangerous thing for my pocket book.
